Troparion of the Feast:

When in the Jordan Thou wast baptised, O Lord, the worship of the Trinity was made manifest. For the voice of the Father bare witness unto Thee, calling Thee His beloved Son, and the Spirit, in the form of a Dove, confirmed the steadfastness of that word. O Christ our God, who didst manifest Thyself and dost enlighten the world: glory to Thee.