What Is the Holy Trinity?
In Deuteronomy, we find the great declaration of the Jewish faith that there is one God (see 6:4), and this affirmation is echoed in Christian teaching (see Jas 2:19). Yet at the same time, Scripture calls three Persons "God" (the Father; the Son, Jesus; and the Holy Spirit) and describes all three in ways that pertain to God alone, not to creatures. Nor are these three names simply various ways of speaking about the same Person: At Jesus‘ baptism, for example, as he (God the Son) emerges from the water, a voice (God the Father) speaks from heaven, and a dove (a form taken by God the Holy Spirit) alights on him (see Lk 3:21-22).
The Church teaches that these two realities of God's "oneness" and "threeness" are not contradictory. Rather, God is one Being in three Persons; within his very essence, he is one community of love.
This Trinity in Unity is no doubt a mystery; after all, as human beings with limited intellects, we can't hope to comprehend fully who God is in himself. Consider this parallel: Someone living in a one-dimensional world, where there can only be points on a line, could not imagine a square. And in a two-dimensional world, where squares exist, a cube would be incomprehensible.
That's how God stands in relation to us. So we accept what he has revealed about himself, even though we find it difficult to fathom. We recognize with the biblical character Job that when we try to figure out God, we are dealing "with great things that [we] do not understand; /things too wonderful for [us], which [we] cannot know" (Jb 42:3).