This is inaccurate. As my dearly beloved
@prodromos @FenderTL5 @Jipsah @MarkRohfrietsch @Yeshua HaDerekh and
@jas3 can confirm, prior to the fourth century the Feast of the Nativity (which is what every church not predominantly English speaking calls Christmas) absolutely was celebrated, but it was celebrated together with the Baptism of Christ on January 6th, and this ancient custom remains in the Armenian Apostolic Church*.
Separating the feasts was logical in response to Arianism, as a means of further stressing the doctrine of the Incarnation*, and the date for the Feast of the Nativity was obvious. At the time the other principle feast of the Incarnation was the Annunciation on March 25th (which for many centuries was used in the former Western Roman Empire as the first day of the civil year, just as September 1st was the start of the civil and ecclesiastical year in the Eastern Roman Empire (and still is the start of the church year among the Eastern Orthodox). So if you take the Annunciation, which is the celebration of St. Gabriel the Archangel announcing to our glorious lady Theotokos and ever virgin Mary that she had been selected to give birth to Christ our True God (see Luke ch. 1 and Matthew ch. 2), to which she consented, becoming glorified as the Mother of God, and add nine months to it, which is the approximate duration of a human pregnancy, you get December 25th.
Since the Feast of the Annunciation substantially predated the fourth century separation of the Feast of the Nativity from the Feast of the Baptism of Christ, we can assert that the feast of the Nativity was dated from the feast of the Annunciation, and not dated based on an attempt to disrupt the Saturnalia (an idea which many historians have pointed out is problematic, and indeed inaccurate with regards to the precise dating of the feast of Sol Invictus and the supposed parallels between them).
The morale of this story is to not rely on centuries old tracts criticizing the Roman Catholic church, particularly on issues where the question is not specific to the Roman Catholic Church, but involves all of the ancient churches, most of which were never under the control of Rome and never accepted Papal Supremacy or Papal Infallibility (indeed, the Oriental Orthodox severed communion with Rome before the Bishop of Rome even adopted the title Pope; for 300 years prior to the adoption of the title Pope in the 530s AD, the title was exclusively used by the Orthodox Pope and Patriarch of Alexandria and All Africa, being held by such illustrious defenders of the Christian faith as St. Athanasius the Great and St. Cyril of Alexandria, as well as martyrs and confessors such as St. Paul of Alexandria, martyred in the Diocletian persecution, and St. Alexander of Alexandria, who was tortured during the Diocletian persecution.
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* Here follows footnotes concerning the Armenians, on the 110th anniversary of the brutal Turkish genocide against the Armenians, which is being commemorated by several Eastern Orthodox churches in addition to the Oriental Orthodox sister churches of the Armenians. Please pray for the Armenians, who once again are threatened by genocide from Azerbaijan, which recently conquered all of the Armenian Republic of Nagorno-Karabakh, and now is threatening to invade and conquer Armenia itself, who they are referring to as “Greater Azerbaijan.”
The Armenian church, newly established by St. Gregory the Illuminator, who was blessed in his efforts to convert the Armenians by an appearance of Christ our True God, on which the Armenian cathedral of Holy Etchmiadzin was built**, did not have major problems with Arianism on the scale of what was being experienced in the Greek, Latin and Syriac churches and also with the conversion of the Visigoths to the false Arian religion (later, many Visigoths, who had settled in North Africa and who had oppressed the Christians, would convert to Islam, which was not a great leap for them, since the main heresy of Arianism, the denial of the Incarnation of our Lord, God and Savior Jesus Christ, the assertion being He was a created being, inferior to and of a different nature from God the Father, is embraced by Islam, which embellishes it by also adding that Jesus Christ is not the Messiah (apparently this is the Mahdi***), but rather just one of the numerous prophets leading up to the supreme prophet Mohammed.
**This being one of the three oldest surviving cathedrals I am aware of (the others being the Hagia Sophia, which the Ottoman Muslims stole from us, and which Ataturk turned into a secular museum, but horrifically Erdogan has resumed the practice of desecrating it with Islamic worship), and the other being the cathedral on the site of the martyrdom of St. Thomas the Apostle in India, which dates to the first century, but unfortunately this was defaced by the Portuguese when they forcibly converted a large number of the Mar Thoma Christians to Roman Catholicism (they had previously been associated with the Church of the East, and a very large number wound up refusing to accept the Roman church and instead became Syriac Orthodox, with a small number later joining the Assyrian Church of the East or the Ancient Church of the East - there is some reason to believe that Christologically, the Church of the East in India was influenced by Nestorius, which is why they converted to Syriac Orthodoxy so smoothly; likewise, being relatively out of touch, they reached out in Syriac Aramaic directly to the Patriarch of Antioch, and by this time sadly the Antiochian Orthodox Church no longer used Syriac Aramaic, (although another dialect of Western Aramaic was and is used in the isolated village of Maaloula), and its doubtful the Antiochian Patriarch could have responde).
*** This poses a bit of a problem for Islam since Muhammed Ahmed al Mahdi, who appeared to meet all of the prophetic qualifications for being the Mahdi, died of typhoid fever three months after brutally conquering the city of Khartoum and massacring all of the Egyptians, including women and children - he was opposed by the eccentric Christian military engineer General Charles Gordon, who had been reinstated as Pasha - viceroy or governor-general of the Sudan, in a desperate attempt by the Khedive to thwart the Mahdi after his army led by the incompetent General Hicks was destroyed by the Mahdi and its modern weapons, including artillery, captured. In many respects, Muhammed Ahmed al Mahdi could be regarded as the prototypical Islamic fundamentalist terrorist.