Among the liturgical ironies of Epiphany is that the date of this ancient feast should be rooted in a heresy and then subjected to the interests of Roman imperial manipulation.
There were many gnostic approaches to Jesus, all tending to assault the integrity of his person. He wasn't human, he only "appeared" to be. He floated through life, his feet barely touching ground. Or, as some had it, the divine spirit swooped down on him at a certain point, occupying his body and slipping away just before the agony of the crucifixion. In short, he never died. Nor was he ever born. Against such the creeds, indeed the scriptures themselves, avail.
In the second century, the Alexandrian followers of one such gnostic, Basilides by name, settled on the baptism of Jesus as the decisive moment of his appearance (epiphaneia) and borrowed an Egyptian solstice festival (Jan. 6) to celebrate the moment of divine manifestation. (I am relying here on the excellent work of Oscar Cullmann in his classic article, "The Origin of Christmas," in The Early Church.) The night before, according to local tradition, the waters of the Nile were reputed to have miraculous qualities. Over against this and the festival of light, they proposed to set the illuminating moment in the Jordan.
The heresy failed. The date and the name stuck.
Before long the orthodox had gotten into the act, bringing the birth narratives along as a lectionary corrective. By early in the fourth century, a Christian festival fully regaled both the baptism and the birth of Jesus on January 6.
Yet this was just the beginning of a struggle. About that same time another Christological controversy arose, a famous one, pitting Arius' view that Christ was but a creature, albeit a lofty one, versus Athanasius', which maintained that Christ was eternally begotten, of one substance with God.