The Holy Trinity is the title given to the three Persons in One God. But how in the world can anyone explain that?
The answer is that we in this world cannot. It is a mystery beyond our comprehension. How can there be three separate persons—The Father, The Son, and The Holy Spirit—but yet still all be the same God?
This is truly a question that we take on faith as Catholics, because our limited human brains simply cannot understand it. And while we cannot fathom this mystery, as Catholics we know and love this fact of three persons in one God.
This most sublime mystery of the faith is stated as: God is absolutely one in nature and essence, and relatively three in Persons (Father, Son and Holy Spirit) who are really distinct from each other, but these Three are consubstantial, that is, identical with the divine substance.
The word “Trinity” does not appear in the Bible, but the doctrine is obvious. Some of our protestant brothers and sisters (who protests against the Catholic Church) use this strategy against Catholics concerning the word “purgatory,” which also does not appear in the Bible, but yet, just as the Trinity is, the doctrine is defined. (8:29)