# if we are unfaithful ... he cannot deny himself

This is most likely the end of a song or poem that Paul is quoting. 

# if we are unfaithful

"even if we fail God" or "even if we do not do what we believe God wants us to do"

# he cannot deny himself

"he must always act according to his character" or "he cannot act in ways that are the opposite of his real character"