SERIES INFO: This series of blog articles will include topics (and bonus sections) I couldn't fit into my book Reintroducing Jesus: Uncovering Jesus of Nazareth in the Misinformation Age.
It may be surprising to us who live in a culture that makes such a big deal about birthdays to learn that several centuries passed before Christians started celebrating the birth of Jesus. The early church focused instead on his resurrection, which makes sense. Everyone has a birthday. Not everyone rises from the dead.
In the second century, pagan critics like Celsus, a Greek philosopher, targeted Christianity and began mocking the virgin birth. The second century also saw the rise of Gnosticism, a religion that combined Christian beliefs with Greek philosophy. Gnosticism denied that Christ had truly taken on flesh; it was only an illusion. He had remained spirit because the material world is, according to Gnostics, wholly evil. At this time, Christian writers started focusing more on the physical birth of Jesus. By the third century, Christian writers had started speculating about when specifically Jesus was born. Some favored dates in May or April, but December 25 and January 6 were dates also proposed.
Today, some place Jesus’ birth in late September or October, saying the shepherds wouldn’t have been outside in the dead of winter with their flocks (Luke 2:8). But could it have been a mild winter? They were, after all, in Israel—not Alaska! In fact, I just checked the weather for Bethlehem since I’m working on this section (appropriately) just days before Christmas. In Bethlehem at night, the temperature is 13 degrees celsius, which is 54 degrees fahrenheit. Not exactly weather for a snowball fight.
In AD 312, Emperor Constantine made Christianity legal in the Roman Empire, and Christians now had the freedom to practice their faith publicly. Jesus’ birth was soon being celebrated. Some Christians opposed it as a holy day, seeing the celebration of birthdays of great men as a pagan custom. The exact year it became widely celebrated is not known, but a document from AD 354 (called Philocalian Chronograph, a sort-of almanac) lists it as a holy day on December 25.
But why December 25th? Here are some often-repeated theories:
As the Roman Empire transitioned from paganism to Christianity, Constantine (or some other civic or religious leader) chose to replace a pagan holiday on December 25 with a Christian holiday.
Before Christianity was legalized in AD 312, Christians would hold their own festivities at the same time as pagan holidays to camouflage their own gatherings. This would certainly make sense during the times Christians came under severe persecution by the Roman government.
Some of the usual suspects for the original December 25 holiday are:
The feast of Saturnalia.
Brumalia, dedicated to Saturn and Bacchus.
The birthday of the Unconquered Sun (Sol Invictus).
The birthday of the Iranian deity Mithra.
The winter solstice.
Now, the big problem with both Theories #1 and #2 is simply this: no evidence. Not that there’s no evidence for pagan festivals around the same time as December 25, but there’s no historical evidence anywhere saying that Christians started celebrating Christmas during this time for one of these reasons.
Some of these pagan festivals, such as the birthday of the Unconquered Sun, were started after Christians began marking December 25 as a special day. Likewise, claims of similarities between the Iranian deity Mithra and Jesus are complete fabrications, including the December 25 virgin birth of Mithra.
To the irk of their pagan neighbors, Christians and Jews have always been stubbornly monotheistic; they believe in only one God and only one true faith. Historian Gerry Bowler tells us that “countless sermons and books by preachers and leaders of the young Church stressed the need to avoid any association with the world of idols and state cults” . Scholar Gregg Allison, author of Historical Theology: An Introduction to Christian Doctrine, says the church in the third and fourth centuries were certainly not open to adopting pagan practices. In fact, at that time the church “denounced any association with paganism and pagan festivals” .
As Christianity spread far and wide over the centuries, it does appear some harmless non-Christian folk customs would intertwine with Christmas. Yet, it’s highly doubtful the devout early church would adopt pagan practices—especially when it was often persecuted for not adopting pagan practices!
Again, no records from that time support any of the above Christmas-as-a-pagan-holiday-replacement theories. To boot, no one proposed this theory until the twelfth century!
CHRISTIAN EXPLANATION #1
Instead, one explanation (which seems odd to us modern folks) is the ancient idea that great men were born and died on the same date. Yet, for whatever reason, the early church decided instead Jesus’ conception and death were on the same day. Since Jesus was crucified during the Passover, that would’ve been March 25 on the Roman calendar. If Jesus was conceived by the Holy Spirit on March 25, nine months later is December 25 .
CHRISTIAN EXPLANATION #2
Another explanation was based on another ancient idea (though not quite as odd as the first one) that the first day God created the earth was springtime. Since to the early church Jesus’ birth was as comparable an event as to the creation of the universe, the angel must have appeared to Mary to announce she was pregnant on March 25. Then, nine months later, Jesus was born in December .
CHRISTIAN EXPLANATION #3
Finally, some calculations for Jesus’ birth resulting in the December 25 date start with the account in Luke’s gospel of John the Baptist’s miraculous conception. John’s birth was foretold by the angel Gabriel when John’s father, Zechariah, was serving in the temple at his priestly division’s appointed time (Luke 1:8, 1:28). The priests were divided into 24 divisions, each serving for a week, two times a year. Zechariah belonged to the eighth division, the division of Abijah (Luke 1:5). By examining the duty roster for the priestly divisions found in the Old Testament (1 Chronicles 24:1–19), some have concluded John was born on June 25. Luke records Jesus’ newly pregnant mother, Mary, visited John’s mother, Elizabeth, in the sixth month of Elizabeth’s pregnancy (Luke 1:36), which would put Jesus’ conception through the Holy Spirit in March. So, December 25 fits once again . But calculating an exact date isn’t so easy. For instance, one big question: When did the priestly division cycles begin?
Historian and Christmas expert, Bowler astutely observes, “For whatever reason the Roman church chose December 25 as the date on which to celebrate the Nativity, it was a momentous decision that would cause centuries of controversy and conflict” .
So, it appears we can’t really take a hard stance on what exact day Jesus was born, but I’ll say it again: this is why Christians should focus not on when they celebrate but why they celebrate.
 Gerry Bowler, Christmas in the Crosshairs: Two Thousand Years of Denouncing and Defending the World's Most Celebrated Holiday, (New York, NY: Oxford University Press, 2017), Kindle, Loc 246.
 Aaron Cline Hanbury, “Fox News Features Allison About the Date of Jesus’ Birth,” Southern Seminary Magazine, Spring 2012, Vol. 80, Num. 2, 6.
 Gerry Bowler, Christmas in the Crosshairs: Two Thousand Years of Denouncing and Defending the World's Most Celebrated Holiday, (New York, NY: Oxford University Press, 2017), Kindle, Loc 256.
 Same as above. Loc 259.
 Same as above. Loc 264.
 Same as above. Loc 269.