Jamieson, Fausset and Brown on 2 Timothy 2:13

13. believe not--"If we are unbelievers (literally, 'unfaithful'), He remains faithful" (De 7:9, 10). The oldest manuscripts read, "For He cannot (it is an impossibility that He should) deny Himself." He cannot be unfaithful to His word that He will deny those who deny Him, though we be not faithful to our profession of faith in Him (Ro 3:3). Three things are impossible to God, to die, to lie, and to be deceived [Augustine, The Creed, 1.1], (He 6:18). This impossibility is not one of infirmity, but of infinite power and majesty. Also, indirectly, comfort is suggested to believers, that He is faithful to His promises to them; at the same time that apostates are shaken out of their self-deceiving fancy, that because they change, Christ similarly may change. A warning to Timothy to be steadfast in the faith.