If we are unbelieving, he remaineth faithful The meaning is, that our base desertion takes nothing from the Son of God or from his glory; because, having everything in himself, he stands in no need of our confession. As if he had said, "Let them desert Christ who will, yet they take nothing from him; for when they perish, he remaineth unchanged." He cannot deny himself This is a still stronger expression. "Christ is not like us, to swerve from his truth." Hence it is evident, that all who deny Christ are disowned by him. And thus he drives away from wicked apostates the flatteries with which they soothe themselves; because, being in the habit of changing their hue, according to circumstances, they would willingly imagine that Christ, in like manner, assumes various forms, and is liable to change; which Paul affirms to be impossible. Yet, at the same time, we must firmly believe what I stated briefly on a former passage, that our faith is founded on the eternal and unchangeable truth of Christ, in order that it may not waver through the unsteadfastness or apostasy of men.

If we believe not, yet he abideth faithful - This cannot mean that, if we live in sin, he will certainly save us, as if he had made any promise to the elect, or formed any purpose that he would save them; whatever might be their conduct; because:
(1) he had just said that if we deny him he will deny us; and,
(2) there is no such promise in the Bible, and no such purpose has been formed. The promise is, that be that is a believer shall be saved, and there is no purpose to save any but such as lead holy lives. The meaning must be, that if we are unbelieving and unfaithful, Christ will remain true to his word, and we cannot hope to be saved. The object of the apostle evidently is, to excite Timothy to fidelity in the performance of duty, and to encourage him to bear trials, by the assurance that we cannot hope to escape if we are not faithful to the cause of the Saviour. This interpretation accords with the design which he had in view.
He cannot deny himself - Implying that it would be a denial of his very nature to save those who are unfaithful. He is holy; and how can he save one who is unholy? His very nature is purity; and how can he save one who has no purity? Let no one, then, suppose that, because he is elected, he is safe, if he lives in sin. The electing purpose of God, indeed, makes salvation sure; but it is only for those who lead righteous lives. Nothing would be mere dishonorable for God than to resolve to save a man that lived habitually in sin; and if that were the doctrine of election, it would deserve all the opprobrium that has ever been heaped upon it.

If we believe not - Should we deny the faith and apostatize, he is the same, as true to his threatenings as to his promises; he cannot deny - act contrary to, himself.

If we believe not, yet he abideth faithful,.... The Syriac and Ethiopic versions read, "if we believe not him". This may be understood, either of such who are altogether destitute of faith, who do not believe in Christ at all; and particularly do not believe what was just now said concerning his denying such that deny him, but mock and scoff at his coming, and at a future judgment: this unbelief of theirs will not make void his faith or faithfulness; see Romans 3:3, he will abide faithful to his word of threatening; and what he says in Mark 16:16 will be found to be an everlasting truth: or it may be understood of true believers, whose faith sometimes is very low, as to its exercise on Christ, and with reference to their future glory and happiness; but Christ is faithful to all his, covenant engagements for them, to bring them to glory, and to every word of promise concerning their happiness, and to every branch of the faithful saying above mentioned; and he is ever the same in his love to them, and in the efficacy of his blood, righteousness, and sacrifice; and his salvation is an everlasting and unchangeable one; nor do the saints' interest in it, and security by it, depend upon their acts of believing, or their frames, but upon the firmness and unchangeableness of Christ, the object of faith.
He cannot deny himself; he cannot go contrary to his word; that would be to act contrary to his nature and perfections, and would be a denying of himself, which is not possible; wherefore his faithfulness will never fail, even though, the faith of his people does, as to the exercise of it.

believe not--"If we are unbelievers (literally, 'unfaithful'), He remains faithful" (Deuteronomy 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 (Romans 3:3). Three things are impossible to God, to die, to lie, and to be deceived [AUGUSTINE, The Creed, 1.1], (Hebrews 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.

If we believe not - That is, though some believe not, God will make good all his promises to them that do believe. He cannot deny himself - His word cannot fail.