What does the Catholic Church teach about the Trinity?

The Catholic Church teaches that God is one divine Being in three distinct, co-equal, co-eternal Persons: Father, Son, and Holy Spirit. Each Person is fully God, sharing one substance, yet genuinely distinct. This central mystery of Christian faith, defined at the Councils of Nicaea and Constantinople, is the source of all other Christian mysteries (CCC 234).

The doctrine is not a human invention but the fruit of God's own self-disclosure in history, reaching its fullness in Jesus Christ and the sending of the Holy Spirit.

One Substance, Three Persons

These distinctions are not divisions. The divine Persons are inseparable in what they are and inseparable in what they do, though each acts according to his personal properties (CCC 267).