"Hear, O Israel: The LORD our God is one LORD" (Deut. 6.4).

"And Jesus, when he was baptized, went up straightway out of the water: and, lo, the heavens were opened unto him, and he saw the Spirit of God descending like a dove, and lighting upon him: And lo a voice from heaven, saying, This is my beloved Son, in whom I am well pleased." (Matt. 3.16-17)

"And it came to pass in those days, that Jesus came from Nazareth of Galilee, and was baptized of John in Jordan. And straightway coming up out of the water, he saw the heavens opened, and the Spirit like a dove descending upon him: And there came a voice from heaven, [saying], Thou art my beloved Son, in whom I am well pleased." (Mark 1.9-11)

"Now when all the people were baptized, it came to pass, that Jesus also being baptized, and praying, the heaven was opened, And the Holy Ghost descended in a bodily shape like a dove upon him, and a voice came from heaven, which said, Thou art my beloved Son; in thee I am well pleased." (Luke 3.21-22)

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 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 exists 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 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 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. 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. There is a word for this...it is called humility.