en_tn/2ti/02/13.md

If we have died with him ... he cannot deny himself

This is most likely a song or poem that Paul is quoting. If your language has a way of indicating that this is poetry you could use it here. If not, you could translate this as regular prose rather than poetry. (See: rc://en/ta/man/translate/writing-poetry)

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"