In the ancient Church, many Christian communities placed little emphasis on the nativity of Christ in Bethlehem, focusing their liturgical attention on the baptism of Christ, the Feast of Theophany. This no doubt resulted from the fact that in the Gospel according to both St. Mark and St. John, the ministry of Jesus Christ and thus the Gospel begins with Christ’s baptism, not with His birth. In all the Gospels the call to repentance and the heralding of the Kingdom begins with St. John the Baptist’s proclaiming his message in the wilderness. Only after His baptism, does Jesus proclaim His message of the Kingdom.
“The role of the Forerunner in the Incarnation and in the redemption of the world can also be clarified from another angle: John is not only the Forerunner but also the Baptist. Although the Nativity of Christ is truly the birth of God and the Most Pure One is truly the Mother of God, God’s becoming man is not yet fully accomplished in the Nativity. It is fully accomplished in the Lord’s Baptism, which is accompanied by the descent of the Holy Spirit upon His human essence; and this is Christ’s Pentecost. After the latter, He becomes truly Christos, the Christ, the one anointed by the Holy Spirit. Just as in humankind fleshly birth is distinguished from that by water and by the spirit, so, for the fullness of divine kenosis and in-humanization, not only the Nativity but also the Baptism of Christ had to occur. In this light, Christ’s Baptism must be viewed as completing the Nativity, and thus it needs a spiritual “birthgiver”, or Baptist. The Baptism is a necessary aspect of the Incarnation. Since the Lord could not baptize Himself without violating the fullness of His in-humanization, He needed the Baptist for this fullness of His Incarnation, just as He needed the Mother for His Nativity.” (Bulgakov, Sergius, The Friend of the Bridegroom: On the Orthodox Veneration of the Forerunner, pg 12)
We can find a similar explanation in the Patristic period in the writings of St. John Chrysostom. St John Chrysostom answered the question “why the baptism of Jesus, and not his birth, is called his epiphany, ie manifestation. [St] John’s answer is that his true nature was not revealed to mankind when he was born, but when the Holy Spirit descended on him at his baptism (cf John 1:26 and 1:33-34).” (Kelly, JND Golden Mouth: The Story of John Chrysostom–Ascetic, Preacher, Bishop, pg 68)