The establishment of the celebration of the Nativity of Christ dates back to the first centuries of Christianity. Until the fourth century, in the Eastern and Western Churches, the feast of the Nativity of Christ was celebrated on January 6, was known as the Epiphany, and initially referred to the actual Baptism of the Savior.
The main and original purpose of establishing the feast is to commemorate and glorify the event of the appearance of the Son of God in the flesh. But there was another reason and purpose for establishing the holiday. Somewhat earlier than in the Orthodox Church, the celebration of Baptism was introduced by Gnostic heretics (Ebionites, Docetitians, Basilidians), because they attached the greatest importance in the life of the Savior to His Baptism. Thus, the Ebionites taught that Jesus was the son of Joseph and the Blessed Virgin Mary and that Christ was united with Him at Baptism; the Docetists recognized human nature in Christ only as illusory.; Finally, the Basilidians did not recognize the incarnation and taught that "God sent forth his Mind, the first outflow of Divinity, and it descended like a dove into the Jordan upon Jesus, who until then had been a simple man accessible to sin" (Clement of Alexandria). But nothing drew Christians into heresy, especially Gnosticism, like the Gnostic worship service, full of harmonious and beautiful songs. It was necessary to contrast the Gnostic holiday with the same one.
And so, the Orthodox Church established a solemn feast of the Epiphany of the Lord and called it Epiphany, suggesting that on this day Christ did not become God for the first time, but only revealed Himself as God, presenting Himself as One of the Trinity, the Son of God in the flesh. In order to undermine the false claims of the Gnostics regarding the Baptism of Christ, the Church began to add to the memory of Baptism the memory of the Nativity of Christ. And thus, in the fourth century, Epiphany and Christmas were celebrated all over the East on the same day, namely January 6, under the common name of Epiphany. The original reason for celebrating Christmas on January 6 (as well as Epiphany) was not the historical correspondence of this date to the birthday of the Lord Jesus Christ, which was not exactly known in ancient times, but a mysterious understanding of the relationship between the first and second Adam, between the culprit of sin and death and the Ruler of life and salvation. The second Adam – Christ, according to the mysterious contemplation of the Ancient Church, was born and died on the same day on which the first Adam was created and died, on the sixth, corresponding to January 6, the first month of the year.
The feast of the Nativity of Christ was first separated from Baptism in the Roman Church in the first half of the fourth century (under Pope Julius). The postponement of the holiday to the 25th and its solemn worship was intended to counterbalance pagan superstitions and thereby turn people's hearts to the knowledge of the true God. It is known that the Romans had a pagan holiday on December 25 in honor of the winter solstice – the day of the appearance of the invincible sun, which winter could not overcome and which from that time goes to spring. This holiday of the renewing "sun god" was a day of unbridled amusements for the people, a day of fun for slaves and children, etc. Thus, this day in itself could not be more appropriate for commemorating the event of the Nativity of Jesus Christ, Who in the New Testament is called the Sun of Truth, the Light of the World, the Savior of people, the Conqueror of death.
The celebration of the Nativity of Christ on December 25 in the Eastern Church was introduced later than in the Western Church, namely in the second half of the fourth century. For the first time, the separate celebration of the Nativity of Christ and the Epiphany of the Lord was introduced in the Church of Constantinople around 377 on the instructions of Emperor Arcadius, according to the custom of the Roman Church and thanks to the energy and power of the eloquence of St. John Chrysostom.