Who introduced the doctrine of Trinity in Christianity and when?

It appears likely that the Gnostic leader, Valentinus, was the first to formulate the concept of the Holy Trinity, early in the second century CE. His work, On the Three Natures, is mentioned by Marcellus of Ancryra, who said that the now-lost work discussed the Father, Son and Holy Ghost as three Persons of the Trinity.

Tertullian, who is believed to have been converted to Christianity by 197 CE, introduced the term Trinity to the Christian vocabulary and also probably the formula "three Persons, one Substance".

The Holy Trinity, as a formal doctrine, was adopted by the Council of Nicaea in 325 CE.

There is an indirect reference to the concept of Trinity in modern Bibles, at 1 John 5:7, but this was never in the early Greek manuscripts, only appearing in the Latin translation of the fifth century, after the Trinity doctrine had been accepted.