It’s not until the 12th century that we find the first suggestion that Jesus’ birth celebration was deliberately set at the time of pagan feasts. A marginal note on a manuscript of the writings of the Syriac biblical commentator Dionysius bar-Salibi states that in ancient times the Christmas holiday was actually shifted from January 6 to December 25 so that it fell on the same date as the pagan Sol Invictus holiday. In the 18th and 19th centuries, Bible scholars spurred on by the new study of comparative religions latched on to this idea.More careful scholarship shows that the dating of Christmas had nothing to do with the Sol Invictus festival and everything to do with the dating of the crucifixion.
Around 200 C.E. Tertullian of Carthage reported the calculation that the 14th of Nisan (the day of the crucifixion according to the Gospel of John) in the year Jesus died was equivalent to March 25 in the Roman (solar) calendar. March 25 is, of course, nine months before December 25; it was later recognized as the Feast of the Annunciation—the commemoration of Jesus’ conception. Thus, Jesus was believed to have been conceived and crucified on the same day of the year. Exactly nine months later, Jesus was born, on December 25.
This also helps to explain why those churches which calculate the date of Easter differently than we do (read Eastern Orthodox) also calculate the date of Christmas to January 6 instead of December 25. But the bottom line is that original Christianity did not think in terms of competition with pagans. They rather thought in terms of the historical suffering, death and resurrection of Jesus.
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