Is it ensure, insure, or assure?
mid-14c., "one who forsakes his religion or faith," from Old French apostate (Modern French apostat) and directly from Late Latin apostata, from Greek apostasia "defection, desertion, rebellion," from apostenai "to defect," literally "to stand off," from apo- "away from" (see apo-) + stenai "to stand." Used in non-religious situations (politics, etc.) from mid-14c.
late 14c.; see apostate (n.).