apostasy Apostasy means renunciation of a religious faith defection. A Muslim may renounce Islam and this is known as apostasy (ridda); or a non-Muslim may embrace Islam and this is called conversion. Under Mohammedan law a person who embraces Islam is imediately governed by Islamic law. But a man who renounces Islam suffers greatly under civil as well as criminal law. Hindu law applied even to those who convert to Hinduism.