What is the X in X-mas?
c.1300, "district with its own church; members of such a church," from Anglo-French paroche, parosse (late 11c.), Old French paroisse, from Late Latin parochia "a diocese," alteration of Late Greek paroikia "a diocese or parish," from paroikos "a sojourner" (in Christian writers), in classical Greek, "neighbor," from para- "near" (see para- (1)) + oikos "house" (see villa).
Sense development unclear, perhaps from "sojourner" as epithet of early Christians as spiritual sojourners in the material world. In early Church writing the word was used in a more general sense than Greek diokesis, though by 13c. they were synonymous. Replaced Old English preostscyr, literally "priest-shire."