The Eastern Orthodox Church fully affirms 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-Constantinopolitan Creed is professed at every Divine Liturgy in its original form, without the Latin addition of 'Filioque' ('and from the Son'). The Orthodox rejection of the Filioque is both theological (the Father alone is the sole Principle of the Holy Spirit's procession) and canonical (the Western Church illegitimately altered the Creed without an Ecumenical Council).

Eastern Orthodox Trinitarian theology draws deeply on the Cappadocian Fathers—St. Basil the Great, St. Gregory of Nyssa, and St. Gregory the Theologian (Nazianzen)—who articulated the distinction between the one divine ousia (essence/substance) and the three hypostaseis (persons). This framework, together with the Palamite distinction between divine essence and divine energies, shapes the Orthodox understanding of how the eternal, unknowable God genuinely communicates himself to creation without compromising his transcendence.