Old English alter, altar "altar," from Latin altare (plural altaria) "high altar, altar for sacrifice to the great gods," perhaps originally meaning "burnt offerings" (compare Latin adolere "to worship, to offer sacrifice, to honor by burning sacrifices to"), but influenced by Latin altus "high."
In Middle English, often auter, from Old French auter. The Latin spelling was restored 1500s. As a symbol of marriage, by 1820. Altar-cloth is from c. 1200. Altar-piece is from 1640s; altar-boy from 1772.