When did Christianity believe in the trinity?

Answer 1

Very early in its formation, Christians believed that Christ & the Holy Spirit as Divine as the Father. However, a few did not, so it caused some rift between Christian groups, within the first few hundred years, until the Council of Nicea, where the Nicene Creed was composed.

Additional Answer:

As the Bible does not teach the doctrine of the Trinity, we can read how it developed gradually after the completion of the New Testament (death of Apostle John) circa 95-100 AD. There were already many heated and controversial ideas attempting to permeate into the Apostolic teachings of Christ. One must consider the influx of Judaism and 'paganism' into the newly formed Christianity. With this surge in numbers into Christian theology came Platonic thought/ideas and phraseology. The term 'trinity' is from the Platonic 'trias' for three and was later Latinized as 'trinitas' which became in English - trinity.

It was succeeding generations of 'Church fathers' who developed this trinity concept, believing they were simply explaining (exegesis) divine revelation and by no means were they 'speculating or inventing' new ideas/thoughts. Yet a close read of first defining the nature of Christ at the Council of Nicea in 325 AD, opening the way for the formulation of the Trinity in the Council of Constantinople in 381 AD will note the 'minority' opinions and novice understanding of Scripture leading this way.

Theology Professor Richard Hanson (Studies in Christian Antiquity, 1985, pp.243-244) noted how the council's decision in 381 AD "was to reduce the meanings of the word 'God' from a very large selection of alternatives to only one" so that "when Western man today says 'God' he means the one, sole exclusive [Trinitarian] God and nothing else." However, a closer read will show how both Emperors Constantine (325) and Theodosius (381) are questionable as to whether they were truly a Christian or barely one showing how unaware or just plain being a 'novice' they were. These two then have decided for many Christians the meaning or definition of the Godhead via the force of the Roman Empire and beyond.