One who deserts his religion

One word substitute is Apostate.

Deserter: a member of the armed forces who deserts.
Turncoat: a person who shifts allegiance from one loyalty or ideal to another, betraying or deserting an original cause by switching to the opposing side or party.
Fanatic: a person filled with excessive and single-minded zeal, especially for an extreme religious or political cause.
Apostate: a person who renounces a religious or political belief or principle.