What is the X in X-mas?
c.1300, "crime of stealing what is consecrated to God," from Old French sacrilege (12c.), from Latin sacrilegium "temple robbery, a stealing of sacred things," from sacrilegus "stealer of sacred things," noun use of adjective, from phrase sacrum legere "to steal sacred things," from sacrum "sacred object" (from neuter singular of sacer "sacred;" see sacred) + legere "take, pick up" (see lecture (n.)). Second element is not from religion. Transferred sense of "profanation of anything held sacred" is attested from late 14c.
originally, the theft of something sacred; as early as the 1st century BC, however, the Latin term for sacrilege came to mean any injury, violation, or profanation of sacred things. Legal punishment for such acts was already sanctioned, in the Levitical code of ancient Israel. The Israelites had extensive rules to safeguard what was holy or consecrated, violation of which (especially of temple laws) often led to mob violence.