It was the church father Tertullian who first used the term ‘Trinity’ for God. That does not mean that Tertullian invented the concept. The idea of the Father, Son, and Holy Spirit is found in the New Testament.
However, it was Tertullian who began to formulate what the New Testament data about these three means within the context of believing in only one God.
As if in this way also one were not All, in that All are of One, by unity (that is) of substance; while the mystery of the dispensation is still guarded, which distributes the Unity into a Trinity, placing in their order the three Persons— the Father, the Son, and the Holy Ghost: three, however, not in condition, but in degree; not in substance, but in form; not in power, but in aspect; yet of one substance, and of one condition, and of one power, inasmuch as He is one God, from whom these degrees and forms and aspects are reckoned, under the name of the Father, and of the Son, and of the Holy Ghost. How they are susceptible of number without division, will be shown as our treatise proceeds. (Against Praxeas 2)
What is very interesting is that this influential text was written after he left orthodox Christianity for the Montanist heresy. This is not what we would expect.
Despite this, what Tertullian began to do here had a great impact on how the Christians of the next century would understand the Trinity.
For more on Tertullian, listen to this episode of the podcast.
