If we believe not. — Better rendered, if we are faithless — that is, untrue to the vows of our Christian profession. The faithlessness implies more than mere unbelief in any of the fundamental doctrines of the faith, such as the Resurrection of the Lord or His divinity.
Yet he abideth faithful: he cannot deny himself. — Those who have understood these words as containing soothing, comforting voices for the sinner, for the faithless Christian who has left his first love, are gravely mistaken. The passage is one of distinct severity — may even be termed one of the sternest in the Book of Life; for it tells how it is impossible even for the pitiful Redeemer to forgive in the future life. “He cannot deny Himself” — cannot treat the faithless as though he were faithful — cannot act as though faithfulness and faithlessness were one and the same thing. The Christian teacher, such as Timothy, and the members of his flock likewise, must remember that, sure and certain as are the promises of glory and happiness to those who love the Lord and try to live His life, so surely will fall the chastisement on all who are faithless and untrue.
With the solemn words of this “faithful saying” St. Paul closes this, the second division of his Epistle — fellowship in the sufferings of Christ here, on this side the grave, and fellowship in the glory of Christ there, on the other side the grave — the one side was the sure consequence of the other; the one could not exist without the other.