Hi, Chris: Actually, they didn't. That's a common misconception, but the early church settled on December 25 centuries before the Roman Catholic Church was a thing (and the Bible tells us the earliest Christians met on the first day of the week--Sunday--to honor the day of Jesus' resurrection). There was a tradition among Jews that famous prophets in Israel were conceived and died on the same date. In the 2nd century, Christians in north Africa calculated (incorrectly) that March 25, 29 A.D. was the date of the Crucifixion. Since that was believed to be the date of his conception as well, you get to December 25 by adding nine months. They got the date wrong, but they arrived at it out of a simple desire to honor the Word made flesh, not by co-opting an existing pagan holiday. In fact, since the feast of Sol Invictus was celebrated in December until the 4th century at the earliest, it's possible that it's the other way around--the pagans tried to steal a holy day from the early church.