"the ox of your foe": Here we find that idolators are called "foes" to Israel in all instance, as in (Devarim 23:10) "When you go out as a troop against your foes," and (Ibid. 21:10) "When you go out to war against your foes." R. Eliezer says: Scripture here speaks of a proselyte who backslid. R. Yitzchak says: It speaks of an Israelite. Why, then, is he called your foe? If he hit your son or quarreled with you he becomes "your foe" for the moment.