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. I can't believe you broke faith with your own sister. She really needed your help tonight! Did you hear? That jerk broke faith with us and backed out of the merger at the eleventh hour!

break/keep ˈfaith with somebody

break/keep a promise that you have made to somebody; stop/continue being loyal to somebody: The government claims they have kept faith with the people by reducing the crime rate.