faithless

adjective

Etymology: From Middle English feithles, equivalent to faith + -less.

Lacking faith; lacking belief in something.
“"You're so faithless about pills," he said, "that I'm not going to give you any."”
Not believing in God, religion, or a comparable ideology.
Unfaithful (said of people, towards their partners)
“Menelaus, who fought to recover his faithless wife, has clearly rooted himself in Sparta for the remainder of his life […]”
Not observant of promises or covenants.
Not true to allegiance, duty, or vows
Serving to disappoint or deceive