Reformed theology, following Nicene orthodoxy, affirms the eternal pre-existence of the Son of God as the second person of the Trinity. The Belgic Confession (Article 10) describes the Son as 'the eternal wisdom of God, the Father's Son,' existing before all creation. The Nicene Creed, which Reformed churches receive, confesses the Son as 'begotten before all ages' and as the one through whom all things were made.

Calvin treated the self-existence (autotheos) of the Son — the Son's possession of divinity from himself rather than as derived from the Father — as crucial for maintaining the Son's full divinity. This was a point of controversy in Calvin's day. The Incarnation, in Reformed theology, is a temporal event in which the eternal, pre-existent Son assumed human nature without any change to his divine nature. The Second Person of the Trinity both continues to sustain the universe by his divine power and simultaneously, in his human nature, lives the life of a Palestinian Jew.