What did "one baptism" in the Nicene Creed meant back then?
Before going on I would like to point out a huge over looked historical fact about the First Council of Nicene of 325: The acts of the Council are not preserved. The Nicene Creed, was approved in its final form at the Council of Constantinople (381), is the profession of the Christian Faith common to the Catholic Church, to all the Eastern Churches separated from Rome, and to many of the Protestant denominations. Pope St. Gregory the Great, following the example of Vigilius and Pelagius II, recognized it as one of the four general councils, but only in its dogmatic utterances.
One baptism would normally encompass a valid and licit baptism of those united in the faith of the Nicene Creed. That said, there would be baptisms of heretics which outside of the true Church would still be valid, yet not in unity with Holy Mother Church. Baptisms must be administered as baptizing someone as doing so with the intention that the Church (“one holy catholic and apostolic Church”) desires in baptizing someone, that is to say in the name of the Holy Trinity to be followed in administering the sacrament of baptism, while at the same time believing in the Most Holy Trinity. Thus St. Athanasius was in favour of denouncing baptism and other sacraments of the Arians as not valid.
Those not believing in the Trinity yet employing a trinitarian formula would not be acceptable to the faithful of the Nicene Creed. This constitutes the mind of the Church in regards to “one baptism”.
The Early Church insisted on the correct matter for baptism which is water 💦 , the Trinitarian formula (I baptize you in the name of the Father, the Son and the Holy Spirit) and to do so with the intentions of the Church to do so.
Some of the Early Fathers, as Tertullian (On Baptism 1) and St. Augustine (Adv. Hær., xlvi and lix) enumerate heretics who rejected water entirely as a constituent of baptism. Such were the Gaians, Manichians, Seleucians, and Hermians. In the Middle Ages, the Waldensians are said to have held the same tenet (Ewald, Contra Walden., vi). Some of the sixteenth century reformers, while accepting water as the ordinary matter of this sacrament, declared that when water could not be had, any liquid could be used in its place. So Luther (Tischr., xvii) and Beza (Ep., ii, ad Till.). It was in consequence of this teaching that certain of the Tridentine canons were framed. Calvin held that the water used in baptism was simply symbolic of the Blood of Christ (Instit., IV, xv).
As a rule, however, those sects which believe in baptism at the present time, recognize water as the necessary matter of the sacrament.
Scripture is so positive in its statements as to the use of true and natural water for baptism that it is difficult to see why it should ever be called in question. Not only have we the explicit words of Christ (John 3:5) "Unless a man be born again of water", etc., but also in the Acts of the Apostles and the Epistles of St. Paul there are passages that preclude any metaphorical interpretation. Thus (Acts 10:47) St. Peter says "Can any man forbid water, that these should not be baptized?" In the eighth chapter of the Acts is narrated the episode of Philip and the eunuch of Ethiopia, and in verse 36 we read: "They came to a certain water; and the eunuch said: See, here is water: what doth hinder me from being baptized?"
Equally positive is the testimony of Christian tradition. Tertullian (On Baptism 1) begins his treatise: "The happy sacrament of our water". Justin Martyr (First Apology, Chapter 1) describes the ceremony of baptism and declares: Then they are led by us to where there is water . . . and then they are laved in the water". St. Augustine positively declares that there is no baptism without water (Tractate 15 on the Gospel of John).
The requisite and sole valid form of baptism is: "I baptize thee (or This person is baptized) in the name of the Father and of the Son and of the Holy Ghost." This was the form given by Christ to His Disciples in the twenty-eighth chapter of St. Matthew's Gospel, as far, at least, as there is question of the invocation of the separate Persons of the Trinity and the expression of the nature of the action performed. For the Latin usage: "I baptize thee", etc., we have the authority of the Council of Trent (Sess. VII, can. iv) and of the Council of Florence in the Decree of Union. In addition we have the constant practice of the whole Western Church. The Latins also recognize as valid the form used by the Greeks: "This servant of Christ is baptized", etc. The Florentine decree acknowledges the validity of this form and it is moreover recognized by the Bull of Leo X, "Accepimus nuper", and of Clement VII, "Provisionis nostrae." Substantially, the Latin and Greek forms are the same, and the Latin Church has never rebaptized Orientals on their return to unity.
In administering this sacrament it is absolutely necessary to use the word "baptize" or its equivalent (Alex. VIII, Prop. damn., xxvii), otherwise the ceremony is invalid.
The mind of the Church as to the necessity of serving the trinitarian formula in this sacrament has been clearly shown by her treatment of baptism conferred by heretics. Any ceremony that did not observe this form has been declared invalid. The Montanists baptized in the name of the Father and the Son and Montanus and Priscilla (St. Basil, Epistle 188). As a consequence, the Council of Laodicea ordered their rebaptism. The Arians at the time of the Council of Nicæa do not seem to have tampered with the baptismal formula, for that Council does not order their rebaptism. When, then, St. Athanasius (Against the Arians, Oration 2) and St. Jerome (Against the Luciferians) declare the Arians to have baptized in the name of the Creator and creatures, they must either refer to their doctrine or to a later changing of the sacramental form. It is well known that the latter was the case with the Spanish Arians and that consequently converts from the sect were rebaptized. The Anomæans, a branch of the Arians, baptized with the formula: "In the name of the uncreated God and in the name of the created Son, and in the name of the Sanctifying Spirit, procreated by the created Son" (Epiphanius, Hær., lxxvii).
Other Arian sects, such as the Eunomians and Aetians, baptized "in the death of Christ". Converts from Sabellianism were ordered by the First Council of Constantinople (can. vii) to be rebaptized because the doctrine of Sabellius that there was but one person in the Trinity had infected their baptismal form. The two sects sprung from Paul of Samosata, who denied Christ's Divinity, likewise conferred invalid baptism. They were the Paulianists and Photinians. Pope Innocent I (Ad. Episc. Maced., vi) declares that these sectaries did not distinguish the Persons of the Trinity when baptizing. The Council of Nicæa (canon 19) ordered the rebaptism of Paulianists, and the Council of Arles (can. xvi and xvii) decreed the same for both Paulianists and Photinians.
Thus the “one baptism” would be a licit and valid baptism using the correct matter, form and intention of the Church to do so with the faith in the Trinity .