Recommended Reading – "The Forgotten Trinity" by James White: https://amzn.to/3Rls3bi (affiliate)

A charge that I have encountered in my comments is that nobody believed in the doctrine of the Trinity prior to the Council of Nicea. There is a halftruth to this in that, in the early days of the Church, the language of Trinitarian doctrine – namely, 1 God in 3 Persons, etc. – was not fully developed yet.

The linguistic understanding of the Trinity that Christians hold today comes largely from the Cappadocian Fathers – Basil the Great, Gregory of Nyssa and Gregory of Nazianzus – who lived around the latter half of the fourth century. Piggybacking off of the work of Athanasius, the great defender of Christ’s deity in the wake of Nicea, the Cappadocian Fathers would give the most detailed expression of what Trinitarianism affirms, that the Father, Son, and Holy Spirit are three coequal, coeternal divine Persons with one common being, substance or nature – that being of God.

It would be anachronistic to look for the fully articulated language of the doctrine of the Trinity prior to the ministry of these men. Nonetheless, what we can look for in early Church is the teaching of the core principles of Trinitarianism; that there is only one God, that Jesus is truly God as the Father is along with the Holy Spirit, but is nevertheless distinct from the Father & Spirit in His Personhood.

One of the preeminent early Church fathers was Ignatius, bishop of Antioch and a student of the apostle John. Ignatius wrote many letters before he was martyred in Rome in the year 107AD. In his letter to the church of Ephesus, Ignatius states in his opening, “…by the will of the Father, and Jesus Christ, our God…” This statement is proof of two things: 1) Ignatius believed Jesus was truly God and 2) Ignatius believed that Jesus was distinct from the Father. He makes a similar statement in chapter 18, while also referring to Jesus as, “the Son of God who was begotten before time began, and established all things according to the will of the Father…” In addition to overtly stating his belief in the deity of Christ, Ignatius also ascribes eternality and Creatorship to Him – these two attributes being ones that are only found in God. In chapter 19, he goes so far as to describe Jesus as, “…God being manifested as a man…”

Ignatius reenforces these beliefs in his letter to the Romans:

“…to the Church which has obtained mercy, through the majesty of the Most High Father, and Jesus Christ, His onlybegotten Son; the Church which is beloved and enlightened by the will of Him that willeth all things which are according to the love of Jesus Christ our God…I also salute in the name of Jesus Christ, the Son of the Father…who are filled inseparably with the grace of God, and are purified from every strange taint, abundance of happiness unblameably, in Jesus Christ our God… to the Church which has obtained mercy, through the majesty of the Most High God the Father, and of Jesus Christ, His onlybegotten Son; the Church which is sanctified and enlightened by the will of God, who formed all things that are according to the faith and love of Jesus Christ, our God and Saviour..."

These statements again affirm that Ignatius is a monotheist (he believes in only one God) while attributing that one essence of God to both the Father & the Son and, at the same time, maintaining a distinction between the Father & Son. This is Trinitarianian belief before the advent of the term Trinity.

“For our God, Jesus Christ, now that He is with the Father, is all the more revealed.”

“There is one Physician who is possessed both of flesh and spirit; both made and not made; God existing in flesh; true life in death; both of Mary and of God; first passible and then impassible, even Jesus Christ our Lord…our Physician is the only true God, the unbegotten and unapproachable, the Lord of all, the Father and Begetter of the onlybegotten Son. We have also as a Physician the Lord our God, Jesus the Christ, the onlybegotten Son and Word, before time began, but who afterwards became also man, of Mary the virgin.”

In this single statement Ignatius affirms: that Christ is truly God, that Christ’s unique office in the Godhead is as the Son and, simultaneously, that Christ is truly man. All of these beliefs are at the core of the Christian faith through the present day.

The Trinity is not a doctrine invented by men or a corrupt Church. It is the clear, consistent teaching of the entirety of the Christian scriptures and it was affirmed as such by the earliest generations of the Church.