The Eastern Orthodox Church teaches that the Holy Spirit proceeds from the Father alone, as the single source and Principle of the Godhead. This is the original teaching of the Nicene-Constantinopolitan Creed ('who proceeds from the Father') and the unanimous consensus of the Eastern Fathers. The Western addition of 'Filioque' ('and from the Son') is condemned by the Orthodox as a heretical innovation introduced unilaterally without an Ecumenical Council, and as theologically problematic: it introduces two principles in the Godhead or subordinates the Spirit to both Father and Son.

The Holy Spirit is fully God, co-equal and co-eternal with the Father and the Son, and is the agent of sanctification, of the Church's life, of the sacramental mysteries, and of theosis. The epiclesis—the solemn invocation of the Holy Spirit over the Eucharistic gifts—is the moment in which the Orthodox believe the transformation of bread and wine takes place (as distinct from the Latin Church's focus on the Words of Institution). Hesychast prayer, as articulated by St. Gregory Palamas, seeks the experience of the Holy Spirit as the uncreated divine light.