faithless

1. Not true to duty or obligation; disloyal.

2. Having no religious faith.

3. Unworthy of faith or trust; unreliable.

Synonyms: These adjectives mean not true to duty or obligation. Faithless and unfaithful imply failure to adhere to promises, obligations, or allegiances: was faithless to her ideals; an unfaithful spouse.
False emphasizes deceitfulness: "To thine own self be true,/And it must follow, as the night the day,/Thou canst not then be false to any man" (Shakespeare).
One who is disloyal betrays an allegiance: disloyal staff members who exposed the senator's indiscretions.
Traitorous most commonly refers to disloyalty to a government or nation: a traitorous double agent.
Treacherous suggests a propensity for betraying trust or faith: "She gave the treacherous impulse time to subside" (Henry James).
Perfidious suggests vileness of behavior and often deceitfulness: a perfidious assassin.