2 Timothy 2:13

εἰ ἀπιοτοῦμεν : It is reasonable to hold that the sense of ἀπιστέω in this place must be determined by the antithesis of πιστὸς μένει. Now πιστός, as applied to God, must mean faithful (Deuteronomy 7:9); one who “keepeth truth for ever” (Psa 146:6; 2 Corinthians 1:18; 1 Thessalonians 5:24; 2 Thessalonians 3:3; Hebrews 10:23; Hebrews 11:11). There is the same contrast in Romans 3:3, “Shall their want of faith (ἀπιστία) make of none effect the faithfulness (πίστιν) of God?” But while we render ἀπιστοῦμεν, with R.V., are faithless, we must remember that unreliability and disbelief in the truth were closely allied in St. Paul's conception of them.

ἀρνήσασθαι γὰρ οὐ δύναται : Being essentially the unchangeable Truth, He cannot be false to His own nature, as we, when ἀπιστοῦμεν, are false to our better nature which has affinity with the Eternal. A lie in word, or unfaithfulness in act, is confessedly only an expedient to meet a temporary difficulty; it involves a disregard of the permanent element in our personality. The more a man realises the transitory nature of created things, and his own kinship with the Eternal, the more unnatural and unnecessary does falsity in word or deed appear to him. It is therefore inconceivable that God should lie (Numbers 23:19; 1 Samuel 15:29; Malachi 3:6; Titus 1:2; Hebrews 6:18). The application of the clause here is not that “He will not break faith with us” (Alf.), but that the consideration of our powerlessness to affect the constancy of God our Father should brace us up to exhibit moral courage, as being His “true children”.