Though this answer is based primarily on the Bible itself, it comes from a perspective outlined in the theological writings of Emanuel Swedenborg (1688-1772).
Human spiritual ages start pristine, then fall, and finally end in corruption and destruction
In the Bible, various time periods or ages of human spiritual history are described in narrative fashion. The general pattern of these ages is that they start in a relatively pristine state, but then decline over time until they become completely corrupted and come to their end. A new period or age then begins, and it, too, goes through a similar cycle.
The earliest human spiritual era
The first such cycle starts with the creation of humankind in Genesis 1 and 2, begins its heavy decline with the Fall of Humankind in Genesis 3, and ends in destruction with the Flood in Genesis 6-9, when the state of the humans of this first period is described thus:
The Lord saw that the wickedness of humankind was great in the earth, and that every inclination of the thoughts of their hearts was only evil continually. (Genesis 6:5)
At its height, this era was characterized by a simple, innocent, heart-based dependence upon God.
Once corrupted, it fell into a state in which the human heart was wholly evil, and the people were unrestrained in their expression of that evil.
The second, clan-based era
The next general age or period is somewhat less sharply defined, but in general it starts after the Flood, as the earth is repopulated from Noah and his family (and the animal population from the animals in the ark). This initiates a time of clans and families.
Though this era does start with Noah and his sons, and the stage is set with the genealogies and the repopulation of the earth recorded in Genesis 10-11, the narrative of this period really gets underway with the stories of Abraham, the first of the three great "Patriarchs" of the Hebrew people. This is the pristine state of this era of clans, and it is often referred back to as such later on in the Bible story.
After Abraham, it goes through a general decline in his sons Isaac and Jacob, until it ends with captivity of Jacob's descendants in Egypt. The Egyptian captivity is the final judgment on this age of clans and families, and brings it to its end.
At its height, this era was characterized by the development of a structured life based on willingly following unwritten codes that prescribed how people were to live within their clans and families, and toward the other clans and families surrounding them. It was the responsibility of the heads of the clans and families to teach and guide family and clan members in observing these codes.
In its later corrupted state, those clan and family codes broke down and were no longer observed. This resulted in brother acting against brother, as in the story of the brothers selling Joseph into captivity, and finally led to the Egyptian captivity.
The third era, of kingdoms, and the kingdom of Israel
What emerges from Egypt is not a clan or family, but a nation. And this whole time period is characterized by nations and empires ruled by kings and emperors rather than by clans and families ruled by heads-of-household and tribal leaders.
In the Bible narrative, the exodus from Egypt begins the Israelite period, which has its pristine time in the desert wanderings and the early conquest of the Holy Land, in which the people follow God in simple (if oftentimes rebellious) fashion. Their worship centers on the Tabernacle, which was a mobile, tent-like sanctuary seen as the dwelling place of God. The Israelite period is often seen as reaching its peak under King David. However, its key figure was Moses, the law-giver of Israel, under whose leadership this period began.
The Israelite era started its heavy decline under David's son and heir, King Solomon. After Solomon's death the nation of Israel split into two competing nations, Israel in the north and Judah in the south. First Israel, then Judah become corrupt and were conquered and taken into captivity by foreign powers. According to the Bible narrative, this was God's judgment on the Israelite nation due to its disobedience and unfaithfulness to God.
Though there was a restoration of Judah and the temple after the Babylonian exile, in the Bible story there was never again a sovereign, independent Israelite kingdom. The Jews in the Holy Land entered a low-level holding pattern that was far from the former glory their nation had achieved under King David. In the Bible story, prophecy ceased for at least two centuries before the coming of Christ.
In short, the Israelite period had an early pristine period under its great lawgiver Moses, a peak under King David, and then declined to its final judgment in captivity and exile, never to re-establish itself as a true nation.
Only a shadow of that ancient Israelite nation existed by the time Christ came, functioning under Roman rule, and often collaborating with its Roman overseers. There was a Temple, and the Jews carried out their sacrificial worship and their religious laws, customs, and traditions. But it was a corrupted version, which Jesus denounced many times in the Gospels. And even this low-level revival of ancient Israelite religion and worship came to a permanent end not long after Jesus' life on earth when in 70 AD the Romans sacked Jerusalem, destroyed the Temple, and dispersed the Jews into many surrounding lands.
At its height, this era was characterized by behavioral obedience to a written code of laws, pronounced at its beginning by God from Mount Sinai, and written down by Moses.
In its later corrupted state, the people had so thoroughly violated those laws that God brought about the destruction of their nation, the captivity of the Israelites, and the disappearance of the ten tribes that had formed the northern kingdom of Israel.
Jesus Christ came when humanity had fallen to its lowest point
The overall picture of the Bible story, then, is one of a pristine beginning, in which the people represented by Adam and Eve lived simply and innocently with God in the Garden of Eden, followed a progressive fall and corruption of humanity through lower and lower spiritual eras, until the time just before the coming of Christ, when the earth was ruled by brutal empires and there was little or no genuine spiritual understanding and spiritual life left on earth.
If we look at the Old Testament narrative as a whole, we find that Jesus Christ came when humankind had reached its lowest ebb, having fallen completely away from its early spiritual state of innocent closeness to God, and successively moved to lower and lower spiritual states until the earth was completely corrupt and unspiritual in God's sight.
At that point, no priest or prophet sent by God could bring about the spiritual salvation and restoration of humankind. Only God himself, coming to earth in human form, had the power to turn the spiritual tide of human history, and begin the long, slow, and painful process of raising humanity back up to the spiritual life and closeness to God for which we were originally created.
Illustrative passages from the Bible
Here are just a few of many passages in the Bible that illustrate this picture. First from the Old Testament prophets:
Isaiah 63:1-6: I have trodden the winepress alone.
The low state to which humanity would fall by the time of Christ is described prophetically in Isaiah 63:1-6:
Who is this who comes from Edom,
in crimsoned garments from Bozrah,
he who is splendid in his apparel,
marching in the greatness of his strength?
"It is I, speaking in righteousness,
mighty to save."
Why is your apparel red,
and your garments like his who treads in the winepress?
"I have trodden the winepress alone,
and from the peoples no one was with me;
I trod them in my anger
and trampled them in my wrath;
their lifeblood spattered on my garments,
and stained all my apparel.
For the day of vengeance was in my heart,
and my year of redemption had come.
I looked, but there was no one to help;
I was appalled, but there was no one to uphold;
so my own arm brought me salvation,
and my wrath upheld me.
I trampled down the peoples in my anger;
I made them drunk in my wrath,
and I poured out their lifeblood on the earth."
It says, "I looked, but there was no one to help . . . so my own arm brought me salvation." In plain language, there was no human being left on earth who could even help in bringing about the salvation of humanity. We had sunk too low. Our religion had become false and corrupt. So the time had come when only God himself could do the job. And he did so by coming as Jesus Christ, to turn the tide of human history, and bring salvation to the people of the earth.
Daniel 2: Nebuchadnezzar's Dream
The prophet Daniel provides two metaphors for this progression of humanity from an early, pristine state to an utterly corrupted and false state.
The first is the story of Nebuchadnezzar's dream of a statue in Daniel 2. The dream itself is described in the words of Daniel himself in Daniel 2:31-35:
You saw, O king, and behold, a great image. This image, mighty and of exceeding brightness, stood before you, and its appearance was frightening. The head of this image was of fine gold, its chest and arms of silver, its middle and thighs of bronze, its legs of iron, its feet partly of iron and partly of clay. As you looked, a stone was cut out by no human hand, and it struck the image on its feet of iron and clay, and broke them in pieces. Then the iron, the clay, the bronze, the silver, and the gold, all together were broken in pieces, and became like the chaff of the summer threshing floors; and the wind carried them away, so that not a trace of them could be found. But the stone that struck the image became a great mountain and filled the whole earth.
Daniel then goes on to interpret the dream as a series of kingdoms, each inferior to the one before it, starting with Nebuchadnezzar's own kingdom, represented by the head of gold, and ending with a kingdom of mixed strength and weakness, represented by the feet partly of iron and partly of clay. Then he says:
And in the days of those kings the God of heaven will set up a kingdom that shall never be destroyed, nor shall the kingdom be left to another people. It shall break in pieces all these kingdoms and bring them to an end, and it shall stand forever. (Daniel 2:44)
Though there have been various attempts to identify the succeeding kingdoms, there is one thing on which Christians agree: the final kingdom that God will set up, represented by the stone that grew into a mountain, represents the kingdom of Christ.
The vision and prophecy in Daniel 2, then, presents a picture of an early "golden age" of innocence and purity, succeeded by successively lower ages represented by silver, bronze, iron, and iron mixed with clay, leading to a final glorious and eternal kingdom, which Christians interpret as the reign of Christ.
Christians may debate the specific meaning of the various kingdoms described in Daniel's interpretation of the statue in Nebuchadnezzar's dream. But overall picture of a pristine beginning and a successive fall to humanity's lowest point, followed by a new, eternal kingdom set up by God himself, is clear.
This scheme of ages symbolized by greater and lesser metals is also reflected in the "Ages of Humankind" in ancient Greek literature. In the Bible, we can correlate the earliest age, represented by Adam and Eve, with the Golden Age, the next age begun with Noah and encompassing the Patriarchs as the Silver Age, and the Israelite era as encompassing the Bronze Age (in its height) and the Iron Age (in its decline), ending in Daniel's age of iron mixed with clay. The kingdom established by Jesus Christ goes beyond the classical Greek scheme of ages, but is reflected in Daniel's vision by the stone that grew into a mountain and filled the whole earth.
Daniel 7: Daniel's Vision of the Four Beasts
Another metaphor provided by Daniel for this succession of lower and lower ages of humankind is his vision of the four beasts in Daniel 7. It follows a pattern similar to Daniel 2, but instead of the imagery of a statue of successively lesser metals, it uses imagery of four progressively fiercer and more terrifying beasts:
- a lion with eagles' wings
- a bear that was raised up on one side, and had three ribs in its teeth
- a leopard with four wings and four heads
- a terrifying, dreadful, exceedingly strong beast with great iron teeth and ten horns
In the end, it provides a similar prophecy of the destruction of all of the kingdoms represented by the four beasts, and the establishment of an eternal kingdom, which Christians also interpret as the reign of Christ. Here is how that kingdom and its king are described in Daniel 7:13-14, 27:
I saw in the night visions,
and behold, with the clouds of heaven
there came one like a son of man,
and he came to the Ancient of Days
and was presented before him.
And to him was given dominion
and glory and a kingdom,
that all peoples, nations, and languages
should serve him;
his dominion is an everlasting dominion,
which shall not pass away,
and his kingdom one
that shall not be destroyed. . . .
And the kingdom and the dominion
and the greatness of the kingdoms under the whole heaven
shall be given to the people of the saints of the Most High;
his kingdom shall be an everlasting kingdom,
and all dominions shall serve and obey him.
Of course, Christians commonly interpret "one like a son of man" as a reference to Jesus Christ, who will rule all the kingdoms of the world in an eternal divine kingdom.
Once again, we see symbolic imagery depicting humans becoming progressively worse, and when the low point of utter corruption is reached, God breaks destroys those former kingdoms and establishes a new, eternal kingdom, which Christians see as the reign of Christ.
Jesus' assessment of the people of his day
In the Gospels, Jesus is fiercely critical of the reigning religious authorities and the spiritual state of humanity that existed in his time. For example, he commonly calls the people of his day "an adulterous generation" (see Matthew 12:39; 16:4; Mark 8:38). His denunciations of the scribes and Pharisees are biting and utterly condemnatory, as seen in Matthew 23, culminating in these words:
"Fill up, then, the measure of your ancestors. You snakes, you brood of vipers! How can you escape being sentenced to hell? Therefore I send you prophets, sages, and scribes, some of whom you will kill and crucify, and some you will flog in your synagogues and pursue from town to town, so that upon you may come all the righteous blood shed on earth, from the blood of righteous Abel to the blood of Zechariah son of Barachiah, whom you murdered between the sanctuary and the altar. Truly I tell you, all this will come upon this generation.
"Jerusalem, Jerusalem, the city that kills the prophets and stones those who are sent to it! How often have I desired to gather your children together as a hen gathers her brood under her wings, and you were not willing! See, your house is left to you, desolate. For I tell you, you will not see me again until you say, 'Blessed is the one who comes in the name of the Lord'" (Matthew 23:32-39, italics added)
Here Jesus sees all of the wickedness of earlier generations culminating in the generation that was currently on earth during his time, and he says that they have reached the point of desolation. So Jesus saw himself as coming at the low point of human spiritual history, when the wickedness of humankind had reached its peak. At that point, only God himself was able to save humankind from total destruction, body and soul.
Emanuel Swedenborg summed it up in this way:
By the time the Lord came into the world the whole planet had completely alienated itself from God by worshiping idols and practicing sorcery; and the church that had existed among the children of Israel and later among the Jews had been utterly destroyed by their falsifying and contaminating the Word. (True Christianity #121:2)
By "the Lord" he means Jesus Christ (see Arcana Coelestia #14). By "the Word" he means the Scriptures, which, for the ancient Jews, would mean the Hebrew Bible.
As long as God could reach people on earth through angels, prophets, and priests, and give them some spiritual life and renewal by those intermediaries, that's precisely what God did, as we see throughout the Old Testament narrative.
But by the time in human history that humanity had fallen to its spiritual low point, having become utterly corrupt and violent, and having falsified all of the spiritual truth that God had provided through angels, priests, and prophets, it was necessary for God himself to come to earth as "the Word made flesh": Jesus Christ.
This was the fullness of time mentioned in the Epistles:
But when the fullness of time had come, God sent his Son. (Galatians 4:4)
He has made known to us the mystery of his will, according to his good pleasure that he set forth in Christ, as a plan for the fullness of time, to gather up all things in him, things in heaven and things on earth. (Ephesians 1:9-10)
Christ came only in the fullness of time, when we humans had fallen so low spiritually that only "God's own arm," Jesus Christ, could save us from the complete destruction to whose brink we had brought ourselves through our long history of falling farther and farther away from God.