The Creed and core doctrine · Question 2
God is one, but the Father, the Son, and the Holy Spirit are three — isn't that a contradiction?
The question: “Isn't that just saying one plus one plus one equals one?”
The Church's answer: The Church does not say that God is one and three in the same sense. It professes that God's nature is one, and that the Father, the Son, and the Holy Spirit are three persons really distinct from one another. So three gods are not dividing the divinity among themselves, and one God is not swapping the masks of Father, Son, and Spirit as the occasion calls for it. The Father is not the Son and the Son is not the Holy Spirit, yet each of the three persons is wholly the one God.
Because this confession keeps two different questions apart — “one nature” and “three persons” — it is not a contradiction in arithmetic. Nor does it mean we have dissected the inner life of God and understood it completely. When the Church calls something a ‘mystery,’ it is not asking you to believe words that don't hang together; it means that God has made this known so that we truly know it, while its depth is more than a finite human being can measure.
Handle the analogies with care
Water, ice, and steam can sound like one God merely changing appearance three times, and a three-leaf clover can sound as though each person were a third of God. A family of three people can call three gods to mind. Such pictures may help you catch hold of one piece, but they don't hold the Trinity accurately. It is safer to remember “one nature, three persons, distinct from one another and yet not divided” than to lean on an analogy.
If they press further: “So since it can't be understood, can I say whatever I like?” — no. Being beyond full measuring and nothing at all being true are two different things. You have to speak inside the boundaries: not three gods, not three masks, and no one person less God than another.
In one sentence: “Catholics don't believe in three gods. We believe in one God, one in nature, in whom the Father, the Son, and the Holy Spirit are three distinct persons.”