【Faithless】 , 【false】 ,  【disloyal】 ,  【traitorous】 ,  【treacherous】 ,  【perfidious】  mean untrue to a person, an institution, or a cause that has a right to expect one's fidelity or allegiance.

【Faithless】 applies to a person, utterance, or act that implies a breach of a vow, a pledge, a sworn obligation, or allegiance. Although often used interchangeably with the strongest of the terms here discriminated, then implying a betrayal of a person or cause, it is also capable of implying untrustworthiness, unreliability, or loss or neglect of an opportunity to prove one's devotion or faith.

【False】  differs from  【faithless】  in its greater emphasis upon a failure to be true or constant in one's devotion or adherence than upon an actual breach of a vow, pledge, sworn promise, or obligation; however it may, like  【faithless】 , carry varying connotations with respect to the gravity or heinousness of that failure.

【Disloyal】 implies lack of faithfulness in thought, in words, or in actions to one (as a friend, superior, sovereign, party, or country) to whom loyalty is owed.

【Traitorous】 implies either actual treason or a serious betrayal of trust or confidence.

【Treacherous】  is of wider application than  【traitorous】 ; as used of persons it implies readiness, or a disposition, to betray trust or confidence <a  【treacherous】 ally>  and as used of things it suggests aptness to lead on to peril or disaster by 【false】 or delusive appearances.

【Perfidious】  is a more contemptuous term than  【treacherous】 ; it implies baseness or vileness as well as an incapacity for faithfulness in the person concerned.