One who abandons his religious faith:

One word-Substitution is Apostate.

Prostate: a gland surrounding the neck of the bladder in male mammals and releasing a fluid component of semen.
Profane: (of a person or their behaviour) not respectful of religious practice; irreverent.
Agnostic: a person who believes that nothing is known or can be known of the existence or nature of God.
Apostate: a person who renounces or abandons a religious or political belief or principle.