As a sometimes militant atheist, I get myself into trouble now and then, and especially at X-Mess time, when I bring the gospel of atheism to social settings.  I haven't done it yet, but I am always tempted to point out that the most likely story, sub rosa, about the so-called "virgin birth" is really simple: At the time, adultery was punishable by death.  Mary would have been stoned to death if no one "bought" the virgin birth.  So they made it up to keep her out of trouble.  Tell that one and you'll be about as welcome as the proverbial turd in a punch bowl.  You can go on, of course, informing the gathered believers that nobody knows the date of Jebus' birth: Constantine had his bishops declare it as December 25th because this was the nativity of the previous deity of choice: Father Mithras.  Ask them if they've noted that Easter pretty much corresponds to the Spring Equinox each year.  All of the Christian feasts are pagan in origin.  The biggest blurt is that there never was anyone called Jesus Christ; there was a legendary Reb Yeshua (Rabbi Joshua), but the Christ part was another invention of Constantine and his bishops.  They may have borrowed it from another pagan deity: Krishna.  Tell the Jebus Crusters they're actually worshipping a Hindu god.  Here endeth my X-Mess epistle.