Eastern Catholic churches fully affirm the doctrine of the Holy Trinity as defined by the Councils of Nicaea (325) and Constantinople (381): one God eternally existing in three co-equal, co-eternal persons—Father, Son, and Holy Spirit. This is held as the foundational truth of Christian faith and is proclaimed in every celebration of the Divine Liturgy.

A distinctive feature of Byzantine Rite practice is that many Eastern Catholic churches recite the Nicene-Constantinopolitan Creed in its original form—without the Latin addition of 'Filioque' ('and from the Son')—while remaining in communion with Rome. This practice was formally acknowledged by the Catholic Church as legitimate for Eastern Catholics, who are not required to adopt the Latin Rite's version of the Creed.