Theophany

'About the beginning of our Lord's thirtieth year, John the Forerunner, who was some six months older than our Saviour according to the flesh, and had lived in the wilderness since his childhood, received a command from God and came into the parts of the Jordan, preaching the baptism of repentance unto the remission of sins. Then our Saviour also came from Galilee to the Jordan, and sought and received baptism though He was the Master and John was but a servant. Whereupon, there came to pass those marvellous deeds, great and beyond nature: the Heavens were opened, the Spirit descended in the form of a dove upon Him that was being baptized, and the voice was heard from the Heavens bearing witness that this was the beloved Son of God, now baptized as a man (Matt. 3:13 17; Mark 1:9 11; Luke 3:1 22). From these events the Divinity of the Lord Jesus Chist and the great mystery of the Trinity were demonstrated. It is also from this that the present feast is called "Theophany," that is, the divine manifestation, God's appearance among men. On this venerable day the sacred mystery of Christian baptism was inaugurated; henceforth also began the saving preaching of the Kingdom of Heaven.' - Great Horologion

When Thou was baptized in the Jordan, O Lord, the worship of the Trinity was made manifest; for the voice of the Father bare witness to Thee, calling Thee His beloved Son. And the Spirit in the form of a dove confirmed the certainty of the word. O Christ our God, Who hast appeared and hast enlightened the world,glory be to Thee. - Troparion of Theophany 

'But Christ's descent into the river has also a further significance. When Christ went down into the waters, not only did he carry us down with Him and make us clean, but He also made clean the nature of the waters themselves... The feast of Theophany has thus a cosmic aspect. The fall of the angelic orders, and after it the fall of man, involved the whole universe. All God's creation was thereby warped and disfigured: to use the symbolism of the liturgical texts, the waters were made a "lair of dragons". Christ came on earth to redeem not only man but through man the entire material creation. When He entered the water, besides effecting by anticipation our rebirth in the font, he likewise effected the cleansing of the waters, their transfiguration into an organ of healing and grace.' - Bishop Kallistos, "Background and meaning of the Feasts" in the Festal Menaion

Source, January 6