break/keep faith with somebody
break faith with (someone or something)
To fail to act as one has promised. After promising so much, the candidate will be in danger of breaking faith with the many groups she courted on the campaign trail.
keep faith with (someone or something)
To maintain loyalty to someone or something; to keep one's promises. Do you trust Ralph? Do you think he'll be able to keep faith with us when people start pressuring him to do the opposite?