What is the definition of the Trinity given by the churches that heard the apostles preach? That ought to tell us something about what the apostles taught.
The earliest actual "statement of faith" that I have found—called "the rule of faith" in those days—is in Against Heresies, written by Irenaeus in A.D. 185.
Irenaeus was not only an orthodox believer; he was one of the most respected Christians of his day. He had sat under the teaching of Polycarp of Smyrna, who had been appointed a bishop by the apostle John.
Irenaeus begins his statement of faith with:
Clearly a faith in one God has always been critical to the Christian faith. The Bible says, "Though there be those that are called gods—whether in heaven or on earth, since there are many gods and many lords—yet for us there is but one God … (1 Cor. 8:5-6).
So there is one God in the Christian faith.
But what does that mean?
There are incorrect views that claim to understand it. Modalism makes its appearance alongside Tertullian's trinitarianism in the 2nd century. Arianism—the belief that the Son is a sort of lesser god, while the Father is the only true God—rears its head in the early 4th century. Both survive to this day, the former in the United Pentecostal and Apostolic churches and the latter in the Jehovah's Witnesses.
But what is the early Christian definition of the Trinity?
Irenaeus' quote continues:
The apostle Paul, in 1 Cor. 8:6, says:
In fact, both the Nicene Creed—the first great ecumenical creed of the Church—and the Apostles Creed, considered a standard of orthodoxy by almost all Christians, begin in precisely the same way:
Yet Tertullian said that it was "the simple" who want to limit God to one person. He spoke quite clearly of a Trinity:
What's up here?
It is well known that the Council of Nicea defended the Trinity against Arius and the Arian party. Nonetheless, their creed says, "We believe in one God, the Father."
Are you ready? In the 2nd century, "the simple" preferred not to think about this. They just became modalists. Nowadays we avoid it by calling it a great mystery.
The early churches didn't. They explained the definition of the Trinity. Do you want their explanation?
Are you up to talking about Greek words like logos and homoousios? Have you ever discussed the substance of God or wondered what God's made of?
I'm so sorry, but there's no other way to talk about the early Christian definition of the Trinity.
For the early churches and even for the apostles, logos was critical. At the Council of Nicea, homoousios was critical. They brought it up, argued about it, and inserted it in the Nicene Creed.
If you're like most Christians, you believe that the Nicene Creed—or its descendant and near relative, the Apostles Creed—is the standard of orthodoxy as a definition of the Trinity.
I do, too.
Their most important issue is that the Son is homoousios with the Father. Do you know what that means?