We say, rightly we cannot fully understand the Trinity; that is not the same as saying we cannot misunderstand it.
The most common early form of misunderstanding, which is still present in some Pentecostal churches is called ‘Modalism’. Modalism is the belief that God, rather than being three persons, is one person who reveals himself in three “modes,” much as an actor might play three roles in a movie. It is also called Sabellianism or monarchianism.
Modalism was the belief of two notable early church figures, Praxeas and Sabellius, both of whom aroused a large following in the church in the late 2nd (Praxeas) and early 3rd centuries (Sabellius). The size of their following and an explanation for it is given by Tertullian in A.D. 200:
The simple, indeed, (I will not call them unwise and unlearned), who always constitute the majority of believers, are startled at the dispensation [of the Trinity], on the ground that their very rule of faith withdraws them from the world’s plurality of gods to the one only true God.
They fail to understand that, although he is the one only God, He must yet be believed in with his own order. The numerical order and distribution of the Trinity they assume to be a division of the Unity, whereas the Unity which derives the Trinity out of its own self is so far from being destroyed, that it is actually supported by it.
They are constantly throwing out against us that we are preachers of two gods and three gods, while they take to themselves the preeminent credit of being worshippers of the one God, as if the Unity itself with irrational deductions did not produce heresy, and the Trinity rationally considered constitute the truth. (Against Praxeas 3)
There is not One Divine Person, there are Three. The earliest definition of Our Faith is to be found in St. Irenaeus:
The Church, though dispersed through our the whole world, even to the ends of the earth, has received from the apostles and their disciples this faith:
[She believes] in one God, the Father Almighty, Maker of heaven, and earth, and the sea, and all things that are in them.
And in one Christ Jesus, the Son of God, who became incarnate for our salvation
And in the Holy Spirit, who proclaimed through the prophets the dispensations of God, and the advents, and the birth from a virgin, and the passion, and the resurrection from the dead, and the ascension into heaven in the flesh of the beloved Christ Jesus, our Lord.
Tertullian shows us the problems the early Fathers had:
Before all things God was alone … He was alone because there was nothing external to him but himself. Yet even then was he not alone, for he had with him that which he possessed in himself—that is to say, his own Reason.
… Although God had not yet sent out his Word, he still had him within himself …
I may therefore without rashness establish that even then, before the creation of the universe, God was not alone, since he had within himself both Reason, and, inherent in Reason, his Word, which he made second to himself by agitating it within Himself.
It is easy to see from this how Arius could conclude: “there was a time when the Son did not exist.”
The early church answer was that there was a time when the Son was not separate from the Father, but there was never a time when he didn’t exist. Before He was separate from the Father, He was already the Logos inside of God. There was a term for this: homoousios. It was so important that it was inserted in the Nicene Creed twice.
It is to that that the second part of this essay will refer.