For Sextus [Julius Africanus] the central issue was the Incarnation, the taking on of flesh by the Son of God. Keeping the anniversary of the creation, he argued that on March 25 Jesus had become incarnate via his conception in his mother’s womb at the annunciation by Gabriel. Following the Jewish exact-dating theory, Sextus believed Jesus had been born precisely nine months later on December 25. This enabled Sextus to keep the sun imagery in an effective way. According to the Julian calendar, December 25 was the winter solstice, the shortest day of the year, when the sun was at its weakest. Every day thereafter it grew stronger and stronger, just as a baby would.
This sounds like we have Christmas, but, as always in scholarship, things were more complicated. Julius was not an influential writer because, while still a pagan, he had served in the army of a persecuting emperor and later worked as a librarian for another emperor. So, many Christians had difficulty accepting him. But at least December 25 had entered the discussion.
Something else had entered the discussion: pagan sun worship. Elagabalus, a member of a Roman dynasty but son of a Syrian, had served as a priest of a Syrian sun god until a palace coup in 218 made him emperor. He introduced into Rome the cult of Deus Sol Invictus (the Unconquered Sun God) to whom he made human sacrifices. In 222, his incompetence and immorality cost him his life via another coup, but he had popularized worship of the sun god which had been growing in Rome before his reign.
The emperor Aurelian (270-275) also worshipped the Unconquered Sun. Realizing that traditional polytheism had declined, he established solar monotheism with a cult centered in Rome itself, although he did nothing to interfere with the multifarious local religions. The great feast-day of the Sun was December 25, the winter solstice and the Sun’s birthday. Initially Aurelian treated the Christians fairly but by 274 he decided to persecute them, likely because Christianity was the only monotheistic religion capable of challenging the worship of Sol Invictus. His murder in 275 by palace officials in a coup prevented the persecution.
Did their challenge to the Unconquered Sun push the Christians toward utilizing December 25 as Christ’s birthday? No Christian author said so explicitly, but it is probable because, where feasible, Christians would replace local pagan traditions with Christian ones, substituting veneration of a local martyr for that of a mythical hero. (This did not always work; many bishops complained about the persistence of pagan traits in Christians celebrations.)
December 25 presented a unique opportunity for Christians to counter three pagan feasts. The cult of the Sol Invictus survived Aurelian. Furthermore, many Roman soldiers and other men venerated a Persian virility deity named Mithra, whose birthday fell on December 25. To this can be added the festival of Saturnalia (December 17 to 23), a week of vigorous drinking, eating, sexual misconduct, and the overturning of social and even gender roles. Celebrating Christ’s birthday on December 25 would directly oppose two pagan feasts and weaken another, as the bishops would have realized. […]
December 25 caught on quickly in the West and within half a century had won favor in the East, reaching Cappadocia by 370, Constantinople by 380, Antioch by 386, and Alexandria by 432. Jerusalem held out, observing the traditional Eastern date of January 6 until circa 575 when the Byzantine emperor Tiberius II (574-582) imposed the new date. Some Eastern Christians, such as the Armenians, still celebrate Christ’s birthday on January 6 but most use it for the Epiphany, the arrival of the Magi. […]
Saturnalia partly survived and transmogrified into the secular revelry of Christmas. In circa 400, bishop Asterius of Amasea in Cappadocia complained that at Christmas people wanted presents so badly that they went into debt; other bishops complained about excessive eating and drinking. Yet the new feast endured.