1580s, "an apostate from a religious faith," probably (with change of suffix) from Spanish renegado (also the form of the English word in Hakluyt, etc.), originally "a Christian turned Muslim," from Medieval Latin renegatus, noun use of past participle of renegare "deny" (see renege).
The general sense of "turncoat, one who deserts to an enemy" is from 1660s. The form renegate, directly from Medieval Latin, is attested in English from late 14c. As an adjective from 1705.