Like Jews and Muslims, Christians are monotheists. In other words, they believe in the existence of precisely one God. Unlike other monotheists, however, Christians also believe that, while there exists just one God, He is three persons - Father, Son, and Holy Spirit. The belief that the one arid only God exists eternally as three persons is known as the doctrine of the Trinity. And this doctrine plays an important role in Christian faith. In fact, the doctrine of the incarnation-which says that Jesus as God became a man and that He is thus both fully divine and fully human-assumes it. This latter doctrine lies at the heart of Christian faith.
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 of the doctrine's claims at 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, and 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 doesn't 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 la 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 persons, and that (2c) there exists only one divine person.
Since 1c entails 1b, affirming both it and la 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 the doctrine is true to entail that it is false. But our inability to understand how God is both one and three tells us far more about ourselves than it does about God. Bible presents God as both one and three; that suffices for us to know that regardless of whether we understand the how of it.
Extracted from the Apologetics Study Bible.