faithless

adjective

1. not adhering to allegiance, promises, vows, or duty: the faithless behavior of Benedict Arnold.
2. not trustworthy; unreliable.
3. without trust or belief.
4. being without religious faith.
5. (among Christians) bereft of Christian faith.

adjective

1. unreliable or treacherous
2. dishonest or disloyal
3. having no faith or trust
4. lacking faith, esp religious faith

adj.

c.1300, “unbelieving,” from faith + -less. Meaning “insincere” is mid-14c. Related: Faithlessly; faithlessness.