The Catholic Church fully affirms the doctrine of the Holy Trinity as defined at the Councils of Nicaea I (325) and Constantinople I (381): one God eternally existing as three co-equal, co-eternal, consubstantial Persons—Father, Son, and Holy Spirit. The Nicene Creed is professed at Sunday Mass and contains the Latin addition 'Filioque' ('and from the Son'), affirming that the Holy Spirit proceeds from both the Father and the Son. This is a defining distinction from Eastern Orthodox practice.

The Catechism of the Catholic Church (§§232–267) presents the Trinity as 'the central mystery of Christian faith and life.' Catholic Trinitarian theology was developed through the Ecumenical Councils and the Latin theological tradition, particularly St. Augustine's De Trinitate and St. Thomas Aquinas's Summa Theologiae, which remain touchstones of Catholic doctrinal reflection. The mystery of the Trinity is understood to be accessible to human reason only through divine revelation.