Apostolic Fathers
The Shepherd of Hermas
the apostles and the teachers who preached the name of the Son of God,
after they had fallen asleep in the power and faith of the Son of God,
preached also to them that had fallen asleep before them (Hermas 93:5)
This passage contains clear, conscious echoes of 1 Peter 3:18-20, but expands upon it to teach that not only Jesus, but His followers, taught the dead in the intermediate state (the subsequent verses indicate this is pre-resurrection). This concept is well-supported by Ante-Nicene Fathers of later generations.
Additionally, in Similitude 8, Hermas gives the parable of the Willow, which depicts the dead (69:6-7 supports the claim that these people are dead, otherwise how had some of them died as martyrs?) being separated into groups prior to receiving judgement and having the opportunity to enter the tower (heaven). For those who were unprepared to enter the tower, a space for repentance was granted. Hermas describes a multi-tiered level of blessings based upon how the individuals took care (or didn't) of the branch they were given.
This is very consistent with an intermediate state--in which not all are on equal footing--prior to final judgement.
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Clement of Rome
Clement supports the contemporary Jewish & Christian view (such as is taught in the parable of the rich man and Lazarus–see Luke 16) that there is an intermediate state in which the righteous receive some form of glory/rest:
4 There was Peter who by reason of unrighteous jealousy endured not
one not one but many labors, and thus having borne his testimony went
to his appointed place of glory.
5 By reason of jealousy and strife Paul by his example pointed out the
prize of patient endurance. After that he had been seven times in
bonds, had been driven into exile, had been stoned, had preached in
the East and in the West, he won the noble renown which was the reward
of his faith,
6 having taught righteousness unto the whole world and having reached
the farthest bounds of the West; and when he had borne his testimony
before the rulers, so he departed from the world and went unto the
holy place, having been found a notable pattern of patient endurance.
(1 Clement 5:4-6)
Here we have an apostolic father who is suggesting that although Peter & Paul have not yet been resurrected (see 1 Clement 24:1 & 26:1), they are already in a better holier place and have already received some form of glory. Clearly Clement believes there is something between death and the resurrection.
Clement's other references to Hades are quotes from the Bible; readers will presumably understand these statements the same way they understand the Bible:
Jealousy brought Dathan and Abiram down alive to hades, because they
made sedition against Moses the servant of God (1 Clement 4:12)
3 For it is good for a man to make confession of his trespasses rather
than to harden his heart, as the heart of those was hardened who made
sedition against Moses the servant of God; whose condemnation was
clearly manifest,
4 for they went down to hades alive, and Death shall be their
shepherd. (1 Clement 51:3-4)
(Both refer to Dathan & his associates--the first passage comes in a long list of Biblical examples of jealousy, without additional commentary by Clement on Hades).
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Polycarp
Polycarp supports (and complicates!) post-mortal consciousness:
I exhort you all, therefore, to yield obedience to the word of
righteousness, and to exercise all patience, such as you have seen
[set] before your eyes, not only in the case of the blessed Ignatius,
and Zosimus, and Rufus, but also in others among yourselves, and in
Paul himself, and the rest of the apostles. [This do] in the assurance
that all these have not run in vain, but in faith and righteousness,
and that they are [now] in their due place in the presence of the Lord
(Epistle to the Philippians, chapter 9)
Polycarp also stated:
To Him [Christ] all things in heaven and on
earth are subject. Him every spirit serves. He comes as the Judge
of the living and the dead (Epistle to the Philippians, chapter 2)
This is interesting, because Polycarp has just been paraphrasing 1 Peter 3:22 & Phil 2:10 (see notes to chapter 2 here), the latter of which reads:
That at the name of Jesus every knee should bow, of things in
heaven, and things in earth, and things under the earth;
Paul says heaven, earth, and under the earth. Polycarp says heaven, earth, and spirit. Considering the extensive Jewish thought on Sheol being located downward or under the earth (e.g. see Numbers 16:32-33), and Polycarp's focus here on the living and the dead, it looks like Polycarp believes that there are dead spirits in Sheol/Hades that are serving the Lord.
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Ignatius
As Ignatius prepared to die a martyr's death, he wrote these words to the church at Rome:
Suffer me to become food for the wild beasts, through whose
instrumentality it will be granted me to attain to God (Romans chapter
4)
Suffer me to obtain pure light: when I have gone thither, I shall
indeed be a man of God (Romans chapter 6)
He believes death is the pathway to God, and he believes he is going somewhere. He does not, unfortunately, give a timeline or a step-by-step description of what he anticipates is to come.
He was truly crucified, and [truly] died, in the sight of beings in
heaven, and on earth, and under the earth (Trallians chapter 9)
Ignatius attests that beings under the earth beheld Christ.
Paul, the holy, the martyred, the deservedly most happy (Ephesians
chapter 12)
Ignatius believes Paul (who is dead) is happy.
The final quote from Ignatius is the most detailed, but is also the only one of my Ignatius quotes whose authenticity is seriously challenged--this may be an editorial addition to Ignatius' words added by a later writer:
He descended, indeed, into Hades alone, but He arose accompanied by a
multitude; and rent asunder that means of separation which had existed
from the beginning of the world, and cast down its partition-wall
(Trallians-long rescension-chapter 9)
There was a separation in Hades, but Jesus was able to go to prison (see 1 Peter 3) and bring out a multitude.
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Epistle to Diognetus
6:8 The soul though itself immortal dwelleth in a mortal tabernacle
Chapter 6 is an extensive contrast between soul & body, including this declaration of the immortality of the soul. Note that "soul" in this chapter is used the way Genesis 2 uses "spirit".
10:7 then shalt thou condemn the deceit and error of the world; when thou
shalt perceive the true life which is in heaven, when thou shalt
despise the apparent death which is here on earth, when thou shalt
fear the real death, which is reserved for those that shall be
condemned to the eternal fire that shall punish those delivered over
to it unto the end.
This suggests that physical death is only "apparent", further supporting the immortality of the soul discussed in chapter 6.
Later Ante-Nicene Fathers
Hippolytus
[T]he just are guided to the right hand, and are led...unto a region of light, in which
the just have dwelt from the beginning of the world. Not constrained
by necessity; but ever enjoying the prospect of the good things they
see, and rejoicing in the expectation of those new enjoyments which
will be peculiar to every one of them: and esteeming those things
beyond what we have here. With whom there is no place of toil; no
burning heat; no piercing cold: nor are any briers there: but the
countenance of the fathers, and of the just, which they see, always
smiles upon them: while they wait for that rest and eternal new life
in heaven, which is to succeed this region. This place we call the
bosom of Abraham.
But as to the unjust, they are dragged by force to the left hand...as prisoners driven by violence...angels...drag them into the neighbourhood of
hell it self...But when they
have a near view of this spectacle, as of a terrible and exceeding
great prospect of fire, they are struck with a fearful expectation of
a future judgment: and in effect punished thereby. And not only so,
but where they see the place [or choir] of the fathers, and of the
just, even hereby are they punished. For a chaos deep and large is
fixed between them. Insomuch that a just man that hath compassion upon
them cannot be admitted; nor can one that is unjust, if he were bold
enough to attempt it, pass over it.
This is the discourse concerning Hades; wherein the souls of all men
are confined, until a proper season; which God hath determined: when
he will make a resurrection of all men from the dead.
(see here; note that this discourse is often mis-attributed to Josephus)
Hippolytus' views align clearly with the parable of the rich man & Lazarus.
--
Irenaeus
The Lord has taught with very great fullness, that souls not only
continue to exist, not by passing from body to body, but that they
preserve the same form [in their separate state] as the body had to
which they were adapted, and that they remember the deeds which they
did in this state of existence, and from which they have now ceased —
in that narrative which is recorded respecting the rich man and that
Lazarus who found repose in the bosom of Abraham...
By these things,
then, it is plainly declared that souls continue to exist that they do
not pass from body to body, that they possess the form of a man, so
that they may be recognised (Against Heresies 2.34.1)
Irenaeus treats the parable of the rich man & Lazarus as a literal statement by the Lord on the realities of the afterlife.
Since, again, some who are reckoned among the orthodox go beyond the
pre-arranged plan for the exaltation of the just, and are ignorant of
the methods by which they are disciplined beforehand for incorruption,
they thus entertain heretical opinions. For the heretics, despising
the handiwork of God, and not admitting the salvation of their flesh,
while they also treat the promise of God contemptuously, and pass
beyond God altogether in the sentiments they form, affirm that
immediately upon their death they shall pass above the heavens...and
go to...that Father whom they have feigned (Against Heresies 5.31.1)
For as the Lord went away in the midst of the shadow of death, where
the souls of the dead were, yet afterwards arose in the body, and
after the resurrection was taken up [into heaven], it is manifest that
the souls of His disciples also, upon whose account the Lord underwent
these things, shall go away into the invisible place allotted to them
by God, and there remain until the resurrection, awaiting that event;
then receiving their bodies, and rising in their entirety, that is
bodily, just as the Lord arose, they shall come thus into the presence
of God (Against Heresies 5.31.2).
Irenaeus affirms the existence of an intermediate state--and that the righteous continue to go there--after the resurrection of Christ--to await their own resurrection.
But the case was, that for three days He dwelt in the place where the
dead were, as the prophet says concerning Him: 'And the Lord remembered
His dead saints who slept formerly in the land of sepulture; and He
descended to them, to rescue and save them.' (Against Heresies
5.31.1)
This statement, attributed to "the prophet", is not found in the currently existing Old Testament. Justin (see below) also quoted this passage, attributed it to Jeremiah, and indicated that the Jews had removed this passage from the writings of Jeremiah.
--
Origen of Alexandria
all the saints who depart from this life will remain in some place
situated on the earth which holy Scripture calls paradise, as in some
place of instruction, and, so to speak, class-room or school of souls,
in which they are to be instructed regarding all the things which they
had seen on earth, and are to receive also some information respecting
things that are to follow in the future...all of which are revealed
more clearly and distinctly to the Saints in their proper time and
place." (De Principiis 2.6)
Those who, departing this world in virtue of that death which is
common to all, are arranged in conformity with their actions and
deserts--according as they shall be deemed worthy--some in the place
which is called 'hell,' others in the bosom of Abraham. (De Principiis
4.23)
In his response to Celsus (an opponent of Christianity):
Celsus next addresses to us the following remark: 'You will not, I
suppose, say of him, that, after failing to gain over those who were
in this world, he went to Hades to gain over those who were there.'
But whether he like it or not, we assert that not only while Jesus was
in the body did He win over not a few persons merely...but also, that
when He became a soul, without the covering of the body, He dwelt
among those souls which were without bodily covering, converting such
of them as were willing to Himself (Contra Celsum 2.43)
--
Tertullian
All souls, therefore, are shut up within Hades...there are already
experienced there punishments and consolations; and there you have a
poor man and a rich...
Why, then, cannot you suppose that the soul undergoes punishment and
consolation in Hades in the interval, while it awaits its alternative
of judgment, in a certain anticipation either of gloom or of glory?
You reply: Because in the judgment of God its matter ought to be sure
and safe, nor should there be any inkling beforehand of the award of
His sentence; and also because (the soul) ought to be covered first by
its vestment of the restored flesh, which, as the partner of its
actions, should be also a sharer in its recompense. What, then, is to
take place in that interval?
Shall we sleep? But souls do not sleep even when men are alive: it is
indeed the business of bodies to sleep, to which also belongs death
itself, no less than its mirror and counterfeit sleep...Do you think
this state is a foretaste of judgment, or its actual commencement? A
premature encroachment on it, or the first course in its full
ministration? Now really, would it not be the highest possible
injustice, even in Hades, if all were to be still well with the guilty
even there, and not well with the righteous even yet? What, would you
have hope be still more confused after death? Would you have it mock
us still more with uncertain expectation? Or shall it now become a
review of past life, and an arranging of judgment, with the inevitable
feeling of a trembling fear? ...
Full well, then, does the soul even in Hades know how to joy and to
sorrow even without the body...
It is therefore quite in keeping with this order of things, that that
part of our nature should be the first to have the recompense and
reward to which they are due on account of its priority. In short,
inasmuch as we understand the prison pointed out in the Gospel to be
Hades, and as we also interpret the uttermost farthing to mean the
very smallest offense which has to be recompensed there before the
resurrection, no one will hesitate to believe that the soul undergoes
in Hades some compensatory discipline, without prejudice to the full
process of the resurrection, when the recompense will be administered
through the flesh besides (A Treatise on the Soul ch. 58)
--
Justin Martyr
The souls of the pious remain in a better place, while those of the
unjust and wicked are in a worse, waiting for the time of judgement.
(Dialogue with Trypho ch. 5)
even after death souls are in a state of sensation (First Apology ch.
18)
from the sayings of the same Jeremiah these have been cut out [by the Jews]: 'The
Lord God remembered His dead people of Israel who lay in the graves;
and He descended to preach to them His own salvation.' (Dialogue with
Trypho ch. 72)
--
Clement of Alexandria
Wherefore the Lord preached the Gospel to those in Hades.
Accordingly the Scripture says, Hades says to Destruction, We have not
seen His form, but we have heard His voice. It is not plainly the
place, which, the words above say, heard the voice, but those who have
been put in Hades, and have abandoned themselves to destruction, as
persons who have thrown themselves voluntarily from a ship into the
sea. They, then, are those that hear the divine power and voice. For
who in his senses can suppose the souls of the righteous and those of sinners in the same condemnation, charging Providence with
injustice?
But how? Do not [the Scriptures] show that the Lord preached the
Gospel to those that perished in the flood, or rather had been
chained, and to those kept in ward and guard? And it has been shown
also, in the second book of the Stromata, that the apostles, following
the Lord, preached the Gospel to those in Hades. For it was requisite,
in my opinion, that as here, so also there, the best of the disciples
should be imitators of the Master; so that He should bring to
repentance those belonging to the Hebrews, and they the Gentiles; that
is, those who had lived in righteousness according to the Law and
Philosophy, who had ended life not perfectly, but sinfully. For it was
suitable to the divine administration, that those possessed of greater
worth in righteousness, and whose life had been pre-eminent, on
repenting of their transgressions, though found in another place, yet
being confessedly of the number of the people of God Almighty, should
be saved, each one according to his individual knowledge.
...it is evident that those, too, who were outside of the Law, having
lived rightly, in consequence of the peculiar nature of the voice,
though they are in Hades and in ward, 1 Peter 3:19 on hearing the
voice of the Lord, whether that of His own person or that acting
through His apostles, with all speed turned and believed...So I think
it is demonstrated that the God being good, and the Lord powerful,
they save with a righteousness and equality which extend to all that
turn to Him, whether here or elsewhere...
Did not the same dispensation obtain in Hades, so that even there, all
the souls, on hearing the proclamation, might either exhibit
repentance, or confess that their punishment was just, because they
believed not? And it were the exercise of no ordinary arbitrariness,
for those who had departed before the advent of the Lord (not having
the Gospel preached to them, and having afforded no ground from
themselves, in consequence of believing or not) to obtain either
salvation or punishment. For it is not right that these should be
condemned without trial, and that those alone who lived after the
advent should have the advantage of the divine righteousness.
(Stromata chapter 6)
Clement believed that the gospel was taught in Hades by Jesus, and later the apostles. Clement shows particular concern for those who never had the opportunity in this life to learn the gospel truth.
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Dissenting views
Tatian
The soul is not in itself immortal...but mortal. Yet it is
possible for it not to die. If, indeed, it knows not the truth, it
dies, and is dissolved with the body, but rises again at last at the
end of the world with the body, receiving death by punishment in
immortality. But, again, if it acquires the knowledge of God, it dies
not, although for a time it be dissolved. (Address to the Greeks ch. 13)
Note, however, the scathing rebuttal of Tatian's credibility by his contemporary, Irenaeus:
They deny, too, the salvation of him who was first created. It is but
lately, however, that this opinion has been invented among them. A
certain man named Tatian first introduced the blasphemy. He was a
hearer of Justin's, and as long as he continued with him he expressed
no such views; but after his martyrdom he separated from the Church,
and, excited and puffed up by the thought of being a teacher, as if he
were superior to others, he composed his own peculiar type of
doctrine. He invented a system of certain invisible Aeons, like the
followers of Valentinus; while, like Marcion and Saturninus, he
declared that marriage was nothing else than corruption and
fornication. But his denial of Adam's salvation was an opinion due
entirely to himself. (Against Heresies 1.28.1)
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Athenagoras
men, in respect of the soul, have from their first origin an
unchangeable continuance, but in respect of the body obtain immortality by means of change. This is what is meant by the doctrine
of the resurrection...
[T]he separation of the soul from the members of the body and the
dissolution of its parts interrupts the continuity of life...For
although the relaxation of the senses and of the physical powers,
which naturally takes place in sleep, seems to interrupt the
sensational life when men sleep at equal intervals of time, and, as it
were, come back to life again, yet we do not refuse to call it life;
and for this reason, I suppose, some call sleep the brother of death,
not as deriving their origin from the same ancestors and fathers, but
because those who are dead and those who sleep are subject to
similar states, as regards at least the stillness and the absence of
all sense of the present or the past, or rather of existence itself
and their own life. (On the Resurrection ch. 16)
This hybrid approach suggests an unending continuance of the spirit, but in a lesser or relaxed state awaiting the resurrection.
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Minucius Felix
In his argument against reincarnation:
But who is so foolish or so brutish as to dare to deny that man, as he
could first of all be formed by God, so can again be re-formed; that
he is nothing after death, and that he was nothing before he began to
exist; and as from nothing it was possible for him to be born, so from
nothing it may be possible for him to be restored?...
Every body, whether it is dried up into dust, or is dissolved into
moisture, or is compressed into ashes, or is attenuated into smoke, is
withdrawn from us, but it is reserved for God in the custody of the
elements...
And I am not ignorant that many, in the consciousness of what they
deserve, rather desire than believe that they shall be nothing after
death; for they would prefer to be altogether extinguished, rather
than to be restored for the purpose of punishment. And their error
also is enhanced, both by the liberty granted them in this life, and
by God's very great patience, whose judgment, the more tardy it is, is
so much the more just. (Octavius ch. 34)
Minucius Felix's arguments show evidence of influence from Tatian (see above).
In the first section he asserts the possibility that if man ceased to exist, God could re-create him. In the second section he offers what today might be called "the law of conservation of energy", that something is never truly destroyed, and in the third he offers a rejection of the "extinguishing" of the dead.
--
Conclusion
Every writer I've cited was born within a century of the Apostolic era.
The early church fathers are nearly unanimous in their belief that Sheol/Hades was a real place where the dead are conscious. They provide abundant support to the view that Hades is divided into (at least) two sections, and offer multiple attestation that 1 Peter 3:18-20, 4:6 speaks of Jesus visiting & teaching the dead in Hades.
Did they believe that it was a real supernatural place that houses the spirits of the dead? Yes, by an overwhelming supermajority.
Did they believe that it was a collective term / metaphor for the set of all the graves of the dead (i.e. just an abstract concept, nothing supernatural)? No.
Did they believe that it was a metaphor for the state of non-being / non-existence of the dead (see Christian mortalism)? This was a very small minority view until later generations.
Did they believe that Abraham's Bosom was a real compartment within Sheol? Yes, this is multiply attested above.
What about Heaven? Did they believe that Christians go to Sheol or Heaven at death? The most direct comment (I have been able to locate) is the statement by Irenaeus emphatically denying that Christians can skip Sheol and go straight to Heaven. This is interesting when juxtaposed with the statement of Polycarp that the martyrs are in the presence of the Lord.
Apostolic Fathers quotations from earlychristianwritings.com
Subsequent Ante-Nicene Fathers quotations from earlychristianwritings.com, newadvent.org, & The Inevitable Apostasy by Tad R. Callister