The Old Catholic Church affirms the doctrine of the Trinity as defined by the Nicene Creed (325 AD) and elaborated in the Chalcedonian Definition (451 AD). God eternally exists as three co-equal, co-eternal persons—Father, Son, and Holy Spirit—united in one divine substance (ousia). Each person is fully God; there is one God, not three.

The Trinity is understood through classical theological language: three hypostases in one ousia, or three persons in one substance. The persons are distinct (not modalism) yet undivided (not tritheism). Each person possesses all divine attributes in their fullness. This doctrine is foundational to Christian faith and worship, reflected in the liturgical doxology and the sacramental theology of the church.

The Old Catholic Church rejects the filioque clause's inclusion in the creed (a bone of contention between East and West), though many Old Catholics do affirm the theology of the Spirit's procession from Father and Son. The church maintains apostolic succession through the Utrecht Union, which itself derives from the ancient Church of Utrecht, and affirms that this ancient faith has been preserved without the innovations of the First Vatican Council.