An apostasy is an abandoning of what you've believed in, whether it's faith, a cause, or principles. This falling away may reflect a simple change of heart or a serious desertion.
What is an apostasy, from the George Bernard Shaw play, Man and Superman?
In George Bernard Shaw's play, Man and Superman, bachelor Jack Tanner feels that he is being forced into marriage and sees himself as a condemned man who has no control over his fate. He explosively denounces marriage:
Marriage is to me apostasy, profanation of the sanctuary of my soul, violation of my manhood, sale of my birthright, shameful surrender, ignominious capitulation, acceptance of defeat. I shall decay like a thing that has served its purpose and is done with; I shall change from a man with a future to a man with a past . . .