Faithless
Faithless , adjective
1.
Not believing; not giving credit.
Be not faithless, but believing.
2.
Not believing on God or religion; specifically, not believing in the Christian religion. — Shakespeare
3.
Not observant of promises or covenants.
4.
Not true to allegiance, duty, or vows; perfidious; trecherous; disloyal; not of true fidelity; inconstant, as a husband or a wife.
A most unnatural and faithless service.
5.
Serving to disappoint or deceive; delusive; unsatisfying.
Yonder faithless phantom.