John 6:69

And we believe,--(See on Mat 16:16). Peter seems to have added this not merely--probably not so much--as an assurance to his Lord of his heart's belief in Him, as for the purpose of fortifying himself and his faithful brethren against that recoil from his Lord's harsh statements which he was probably struggling against with difficulty at that moment. Note.--There are seasons when one's faith is tried to the utmost, particularly by speculative difficulties; the spiritual eye then swims, and all truth seems ready to depart from us. At such seasons, a clear perception that to abandon the faith of Christ is to face black desolation, ruin and death; and on recoiling from this, to be able to fall back, not merely on first principles and immovable foundations, but on personal experience of a Living Lord in whom all truth is wrapt up and made flesh for our very benefit--this is a relief unspeakable. Under that blessed Wing taking shelter, until we are again fit to grapple with the questions that have staggered us, we at length either find our way through them, or attain to a calm satisfaction in the discovery that they lie beyond the limits of present apprehension.

— Jamieson-Fausset-Brown Commentary (public domain)