Trinitarianism is the Christian doctrine that God, although one being, exists in three distinct persons (hypostases) known collectively as the Holy Trinity. Trinitarianism was formally defined in fourth-century Christian ecumenical councils. The Councils met at a time when Christians radically disagreed, to the point of bloodshed, over the divine nature of Christ- whether Jesus was divine, whether God created Jesus, whether Jesus therefore was not God, but a distinct being, etc. The Byzantine Emperor in 325 convened an election to come up with a solution and remove dissent. Trinitarianism was postulated by Athanasius as a response to the Jesus is God/ Jesus Had Always Been God logical conundrum. The Council of Nicaea, in a controversial election marked by violence and intimidation, voted to adopt the notion of the Holy Trinity as an article of faith (creed). Other ideas, such as Arianism, regarding the nature of the Trinity and the nature of Christ within it, were declared heretical.

While its detractors believe it was invented in the fourth century when it was first expressly stated, adherents of Trinitarianism believe that the doctrine was nevertheless taught by Jesus Christ and the first apostles, being indicated throughout the Bible and foreshadowed in the Old Testament specifically.

Many Trinitarian Christians believe that the orthodox doctrine of the Trinity is so central to the Christian faith that to deny it is to reject the Christian faith entirely. They accordingly consider nontrinitarian or antitrinitarian groups to be non-Christian, even though such groups identify themselves as Christian. The nature of the dispute tends to revolve chiefly over the issues of the deity of Jesus and whether belief in a non-divine Jesus is sufficient to confer salvation.

Some have argued that the trinity was of pagan origin, and viewed trinitarian doctrines as a contamination of the Christian faith. Most of the ideas of one such author are described in his book The Two Babylons. That said, the vast majority of theologians, within several hundred years after the Council of Nicaea, affirm that the doctrine of the Trinity is a central tenet of the Christian faith. Today, most mainstream Christian denominations are Trinitarian. People who believe in oneness incorrectly believe that those who believe in the Trinity believe in 3 Gods.