The Weakness of Human Resolutions
One would have thought that Peter, upon the warning of a denial, should have begun to tremble, and not to boast; to arm himself and not presume, to suspect his strength, and not to promise. But a double warning (Matthew 26:31, 34) finds in Peter a double presumption which makes him confident, believing that even naked and empty human nature, with such an invincible resolution, if lacking courage, would have enough shame to persist. If the resolution is broken it would imply a weak and inconsistent spirit more faithless in the execution of the promise and impotent in its contempt of death than the courage and honesty of Peter could admit to.
It is the justice of God to give man over to fainting and falling when he relies on himself. One tear or sigh, though signs of weakness, could have prevailed more to strengthen Peter’s faith, than so many fruitless boasts, the gildings and flourishing of a rotten confidence. A little pebble stone will overturn and sink down a Goliath, when all the armour of Saul will more hinder than profit in such a conflict (1 Samuel 17:29ff).