The term "apostate" refers to someone who abandons or renounces their religious faith or beliefs.
This term specifically addresses the act of desertion in a religious context.
"Deserter" (1) may refer to someone who abandons any duty or allegiance, not specifically religion.
"Opportunist" (3) refers to someone who takes advantage of situations for personal gain, often without concern for principles, but doesn't necessarily involve desertion of beliefs.
"Reticent" (4) means someone who is reserved or silent, which is unrelated to the concept of abandoning one's religion.
The most accurate term in this context is "apostate," as it specifically refers to a person who has abandoned their faith.
Conclusion:
An "apostate" is someone who has deserted their religion or belief system, making it the correct term for this expression.