How celebrating Christmas protected the doctring of the Church of God becoming Man -Incarnation

This is a snippet from the site, way of the (Church) fathers

In the earliest days of the Church, Christmas was not one of the important feasts. Jesus’ life was still a living memory, and His extraordinary resurrection rightly occupied the central spot in the calendar. But as time went on, false teachers began to deny the fact of Jesus’ humanity. They claimed that Jesus’ body had been an elaborate disguise, that, in reality, God had never debased Himself by taking on human flesh. Later heretics denied also that Mary gave birth to the Word: instead, they said, she gave birth to a “vessel” into which the Word was later poured. Still other heretics believed that the Son was a subordinate being — divine, but not coeternal with God the Father.

All these heresies had one thing in common: an unwillingness to face the apparent foolishness of the Incarnation. Arius, the founder of the Arian heresy, was an eminently reasonable man. He denied the doctrine of the Trinity because, he said, three cannot be one; that’s elementary arithmetic. The infinite God cannot become finite man; that’s elementary philosophy. Therefore there could be no Incarnation.

Heretics like Arius wanted to spare God the unreasonable indignity of being corrupted by too close an association with humanity. It was the same problem the Pharisees could not get over: If this Jesus is so good, why does He associate with sinners and tax collectors? In fact, though the heretics would have insisted that they were defending the perfection of the Deity, they were actually denying the perfection of God’s love. Love, after all, can seem unreasonable. Anyone who values another as much as oneself seems entirely unreasonable.

It can hardly be coincidence that the celebration of the literal, historical birth of Jesus the carpenter’s son began to take on more importance just when the true faith was most dangerously beset by these flesh-denying errors. The scandalously human birth of the Son of God was the very thing that separated orthodoxy from heresy. Celebrating that Nativity committed the Church to a clear statement of principle.