On its face, however, the doctrine of the Trinity might look like a contradiction. It might seem impossible that God be both one and three. Indeed, the apparent absurdity of this doctrine has led to at least two major errors, each of which elevates one the doctrine’s claims at the expense of the other’s expense. On one hand, some stress the oneness of God at the expense of His threeness, claiming there is only one divine person. Those who describe God in this way usually say that the one divine person appears in different guises or masks, sometimes as Father, other times as Son, and still other times as Spirit. Since this view says the one divine person changes His mode of appearance, it is called “modalism.” On the other hand, some stress God’s threeness at the expense of His oneness, claiming each of the three divine persons is a distinct god. This view, which says that there are three gods, is called “tritheism.”
But modalism and tritheism are at odds with the Bible, which presents God as both one and three. There is just one God (Dt 6:4), yet this God is three persons—Father, Son, Spirit (Mt. 3:16-17; Mk 1:9-11; Lk 3:21-22). No doubt it is difficult (or perhaps even impossible) for us to understand how God is both one and three. But something’s being difficult (or even impossible) for humans to understand does not make it a contradiction.
A contradiction involves saying that something is both true and false at the same time and in the same way. So, for instance, one who says both that Napoleon lost the Battle of Waterloo and that Napoleon did not lose the Battle of Waterloo contradicts himself. It is logically impossible for Napoleon to have both lost that battle and not to have lost it. His claim is contradictory.
Now if Christians said both that
(1a) there exists precisely one God, and that
(1b) it is not the case that there exists precisely one God, they would contradict themselves.
So also if they said both that
(2a) there are three divine persons, and that
(2b) it is not the case that there are three divine persons, they also would contradict themselves.
But Christians do not affirm both 1a and 1b. Neither do they affirm both 2a and 2b. Rather, they affirm 1a and 2a. And this would be contradictory only if either 1a entails 2b or 2a entails 1b.
To put the point differently, when Christians say that God is both one and three, they do not say that He is one in the same way in which He is three. So, for instance, they do not say both that (1a) there exists precisely one God, and that (1c) there exist three gods. Nor do they say both that (2a) there exist three divine person, and that (2c) there exists only one divine person.
Since 1c entails 1b, affirming both it and 1a would be contradictory. And since 2c entails 2b, affirming both it and 2a also would be contradictory. But, as a matter of fact, Christians deny both 1c and 2c. In affirming 1a and 2a, then, Christians affirm that in one way God is one and in another way He is three. And in so doing they do not contradict themselves.
So, then, those who think the doctrine of the Trinity is contradictory misunderstand either the nature of a contradiction or the doctrine itself. Perhaps they confuse contradiction with mere paradox, taking our inability to understand how God is both one and three tells us far more about ourselves than it does about god. The Bible presents God as both one and three; that suffices for us to know that He is so, regardless of whether we understand the how of it.