Reformed theology unreservedly affirms the doctrine of the Trinity as expressed in the Nicene and Athanasian Creeds. The Heidelberg Catechism (Q&A 25) confesses one divine being in three persons — Father, Son, and Holy Spirit — equal in power, glory, and eternity. The Belgic Confession (Articles 1 and 8–9) provides an extended treatment of the Trinity, affirming the eternal generation of the Son and the eternal procession of the Spirit.

Calvin devoted extensive attention to the Trinity in the Institutes of the Christian Religion (Book I, chapters 13–14), defending Nicene orthodoxy against anti-Trinitarian opponents such as Michael Servetus. Reformed orthodoxy also developed the doctrine of the eternal generation of the Son and, through the work of theologians like Francis Turretin, contributed to sophisticated Trinitarian formulation. The Trinity undergirds all Reformed theology: creation, covenant, election, and redemption are understood as the cooperative work of the triune God.