Some reference sources will tell you that Christians have always believed in the Trinity. This claim is misleading at best. Rather than dating trinitarian theology to the start of Christianity, if we carefully examine the history of theology, we can see a relevant series of dates, as elements of belief in a triune god emerge.
Yes, Christians have always believed in one God, but in all mainstream creeds in the first three centuries, this is identified with the Father. For instance, Irenaeus (c. 135-200) asserts between 182 and 188 that all Christians have always believed in “One God, the Father Almighty, Maker of heaven, and earth, and the sea, and all things that are in them.” (Against Heresies 1.10) And even the creed from Nicea in 325 continues this tradition: “We believe in one God the Father all powerful…”
It is, frankly, an apologetic spin to assert that Christians have always believed in the Trinity, that is, in a tripersonal god. Often, a person who says this is confusing the Trinity with the trinity. Talk of the trinity (Greek: trias) enters into catholic tradition around 185. But this doesn’t suggest belief in the Trinity; as their creeds and other writings show, mainstream Christians c. 150-350 continue to identify the one God with the Father (only).
How early did Christians believe in a Trinity, in a tripersonal god, in whom there are three equally divine, or nature-sharing “Persons”? An obvious sign of such belief would be talking about the one God as the Trinity, and not, as earlier, the Father. For instance, Augustine (354-430) writes in 393 that “this Trinity is one God, according as it is written, ‘Hear, O Israel, the Lord your God is one God.’” (On Faith and the Creed 9) In Augustine’s view, the one god Yahweh just is the Trinity, and vice-versa.
What’s the earliest such usage of “Trinity”? The earliest usage I have found so far is from the year 385, by Gregory of Nyssa. (Great Catechism 39) But the idea of the one God as the Trinity is implied in many works of the 370s and 380s.
Early on, Christians did not call the Holy Spirit “God,” nor did they worship or pray to the Holy Spirit. Why? Worship and prayer to the Holy Spirit simply don’t occur in the New Testament. And it is a fraught question how to understand talk of “the holy spirit” there. In a famous incident in Acts 5, the apostle Peter confronts Ananias and then his wife Sapphira about their lies. To the husband, Peter says,
“Ananias, how is it that Satan has so filled your heart that you have lied to the Holy Spirit and have kept for yourself some of the money… You have not lied just to human beings but to God.”
And to the wife,
“How could you conspire to test the Spirit of the Lord?”
Does Peter here presuppose that to pneuma hagion (“the holy spirit”), that is, to pneuma kuriou (“the spirit of the Lord”) is a Person within the triune god? Frankly, that seems an anchronistic projection back into the text. We can instead just take phrases as a way of referring to God, the God who is active and present in Peter. This is the one, God, to whom they have lied (in addition to Peter and the other apostles).
Logically, for Christians to be trinitarian, in the sense defined at Constantinople (381), they have to believe certain things. One is that the Son is divine, and just as divine as the Father (“true God from true God”), not in some lesser way. Another, relatedly, is that Son is eternal, that he never came into existence, never began to exist. The same two points must hold regarding the Holy Spirit; he must be held to be fully divine, as the Father is divine, and he must always have existed (or must exist timelessly); he can’t have come into existence.
Mainstream, non-monarchian, catholic theology c. 150-225 fails all four conditions. Because of this, it undeniably fails to be trinitarian, even though it does talk of the trinity. Let’s look at two champions of catholic orthodoxy at the end of this era, Tertullian, and Origen.
Arguing against the gnostic Hermogenes, Tertullian explicitly asserts that the Father is older, nobler, stronger, and more powerful than the Son. Quoting Proverbs 8, he insists that
“….the very Wisdom of God [i.e. the pre-human Jesus] is declared to be born and created, for the especial reason that we should not suppose that there is any other being than God alone [i.e. the Father] who is unbegotten and uncreated. For if that which from its being inherent in the Lord was of Him and in Him, was yet not without a beginning, – I mean His wisdom, which was then born and created, when in the thought of God It began to assume motion for the arrangement of His creative works…” (Against Hermogenes ch. 18, p. 487)
This is what the patristic scholar Wolfson calls a “two stage” logos theory. Eternally, God is wise, so his “wisdom” is eternal. But a finite time ago, when it was time to create, the theory is that God “uttered” his “word” or “wisdom,” that is, he as it were made his eternal wisdom to be a helper, through whom he will create. (See Against Praxeas ch. 6-7 for another description of this.) This is to say, if by the Logos/Word/Wisdom you mean a divine self, this came into existence a finite time ago, though God has always had Wisdom (etc.) as property in himself. This is pretty clear in all of his works. (See further my “Tertullian the Unitarian,” forthcoming.) The point is that for this major champion of catholic theology in the years 197-225, the Son is neither eternal nor as divine as the Father. Nor is Tertullian an anomaly. The two-stage logos theory is standard in this era, c. 150-250.