Augustine of Hippo — as excerpted in the Catena Aurea on John 6:60-71

“(Tr. xxvii. 7) So then (our) faith is given to us: and no small gift it is. Wherefore rejoice if thou believest; but be not lifted up, for what hast thou which thou didst not receive? (1 Cor. 4:7.) And that this grace is given to some, and not to others, no one can doubt, without going against the plainest declarations of Scripture. As for the question, why it is not given to all, this cannot disquiet the believer, who knows that in consequence of the sin of one man, all are justly liable to condemnation; and that no blame could attach to God, even if none were pardoned; it being of His great mercy only that so many are. And why He pardons one rather than another, rests with Him, whose judgments are unsearchable, and His ways past finding out.”