Trinitarianism is the Christian doctrine that God, although being one, exists in three distinct persons (hypostases) known collectively as the Holy Trinity. Trinitarianism was formally defined in fourth-century Christian ecumenical councils that adopted the notion of the Holy Trinity as an article of faith in reaction to certain ideas, such as Arianism, regarding the nature of the Trinity and the nature of Christ within it, which were declared heretical. Adherents of the doctrine believe it was taught by Jesus Christ and the first apostles, and was foreshadowed in the Old Testament, while its detractors believe it was invented in the fourth century.

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 scholars, most notably, Alexander Hislop defended the idea that the trinity was of pagan origin, and viewed trinitarian doctrines as a contamination of the Christian faith. Most of his ideas are described in his book The Two Babylons. That said, the vast majority of theologians, both ancient and modern, affirm that the doctrine of the Trinity has traditionally been a central tenet of the Christian faith. Today, all mainstream Christian denominations are Trinitarian.