John 6:70

70.Jesus answered them. Since Christ replies to all, we infer from it that all spake by the mouth of Peter. Besides, Christ now prepares and fortifies the eleven apostles against a new offense which was already at hand. It was a powerful instrument of Satan for shaking their faith, when they were reduced to so small a number, but the fall of Judas might take away all their courage; for since Christ had chosen that sacred number, who would ever have thought that any portion of the whole number could be torn away? That admonition of Christ may be interpreted thus: “You twelve alone remain out of a large company. If your faith has not been shaken by the unbelief of many, prepare for a new contest; for this company, though small, will be still diminished by one man.”

Have not I chosen you twelve? When Christ says that he has chosen or elected twelve, he does not refer to the eternal purpose of God; for it is impossible that any one of those who have been predestinated to life shall fall away; but, having been chosen to the apostolic office, they ought to have surpassed others in piety and holiness. He used the word chosen, therefore, to denote those who were eminent and distinguished from the ordinary rank.

And one of you is a devil. He unquestionably intended, by this name, to hold up Judas to the utmost detestation; for they are mistaken who extenuate the atrocity implied in the name and indeed we cannot sufficiently execrate those who dishonor so sacred an office. Teachers who faithfully discharge their office are called angels

They should seek the law at his mouth, for he is the angel of the Lord of Hosts,
(Malachi 2:7.)

Justly, therefore, is he accounted a devil, who, after having been admitted to so honorable a rank, is corrupted through his treachery and wickedness. Another reason is, that God allows more power and liberty to Satan over wicked and ungodly ministers, than over other ordinary men; and therefore, if they who were chosen to be pastors are driven by diabolical rage, so as to resemble wild and monstrous beasts, so far are we from being entitled, on that account, to despise the honorable rank to which they belong, that we ought rather to honor it the more, when the profanation of it is followed by so fearful a punishment.