For this feast of the Baptism of Jesus, we have part of a sermon on the topic by St Gregory of Nazianzus (330-390), also known as Gregory the Theologian. (Oratio 39 in sancta Lumina, 14-16. 20: PG 36, 350-351, 354, 358-359).

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Christ is bathed in light; let us also be bathed in light. Christ is baptized; let us also go down with him, and rise with him.

John is baptizing when Jesus draws near. Perhaps he comes to sanctify his baptizer; certainly he comes to bury sinful humanity in the waters. He comes to sanctify the Jordan for our sake and in readiness for us; he who is spirit and flesh comes to begin a new creation through the Spirit and water.

The Baptist protests; Jesus insists. Then John says: I ought to be baptized by you. He is the lamp in the presence of the sun, the voice in the presence of the Word, the friend in the presence of the Bridegroom, the greatest of all born of woman in the presence of the firstborn of all creation, the one who leapt in his mother's womb in the presence of him who was adored in the womb, the forerunner and future forerunner in the presence of him who has already come and is to come again. I ought to be baptized by you; we should also add: and for you, for John is to be baptized in blood, washed clean like Peter, not only by the washing of his feet.

Jesus rises from the waters; the world rises with him. The heavens like Paradise with its flaming sword, closed by Adam for himself and his descendants, are rent open. The Spirit comes to him as to an equal, bearing witness to his Godhead. A voice bears witness to him from heaven, his place of origin. The Spirit descends in bodily form like the dove that so long ago announced the ending of the flood and so gives honor to the body that is one with God.

Today let us do honor to Christ's baptism and celebrate this feast in holiness. Be cleansed entirely and continue to be cleansed. Nothing gives such pleasure to God as the conversion and salvation of men, for whom his every word and every revelation exists. He wants you to become a living force for all mankind, lights shining in the world. You are to be radiant lights as you stand beside Christ, the great light, bathed in the glory of him who is the light of heaven. You are to enjoy more and more the pure and dazzling light of the Trinity, as now you have received--though not in its fullness--a ray of its splendor, proceeding from the one God, in Christ Jesus our Lord, to whom be glory and power for ever and ever. Amen.