en_tn_condensed/2ti/02/13.md

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"