Jn. 6:69. And we have believed and have come to know that You are the Christ, the Son of the living God.
Peter, having a tender love for Him and for the brethren, answers on behalf of the entire company of the apostles. He did not say "Lord! to whom shall I go?" and "I have known and believed," but "to whom shall we go?" and so forth.
He says that the Lord has the words of eternal life, because he had heard Him say: "I will raise up the one who believes in Me, and he will have eternal life." For they had already accepted the truth of the resurrection and all the teaching. Therefore, the cause of the offense was not the words of Christ, but the inattentiveness, negligence, and ingratitude of the listeners. For these twelve also heard the same things, and yet were not offended, but remained faithful.
Jn. 6:70. Jesus answered them: Did I not choose twelve of you? Yet one of you is a devil.
What then does Christ say? Does He approve Peter, as He did on another occasion (Matt. 16:17)? Not at all, so as not to appear to be flattering him for having remained with Him. What then does He say to Peter? "I chose you twelve, yet not all of you are worthy of this choosing, but one of you is a devil." This can be understood in two ways: either he is a devil because even now he perverts everything of Mine and accepts nothing of it, or because he will slander Me. For such indeed Judas proved to be, slandering the Lord before the Jews and betraying His secrets to them, as David also says of him: "he went abroad and spoke together" — he went out and spoke in concert (Ps. 41:6). The Lord says this to Peter in order to correct him. Peter said: "we have come to know and have believed." But He says: "do not think so; you are not all faithful, nor have you all come to know Me."
Look, perhaps, how the Lord from afar forestalls the malice of the betrayer and, as if forcibly restraining it, says this, although He knows it will bring no benefit, yet He fulfills His own task. And He neither exposed him nor left it completely unknown; the first He did so that the betrayer, having lost all shame, would not become more obstinate; the other, so that he would not commit the bold deed without fear, and would not think that he would remain hidden. From this we also learn that God does not make people good by compulsion and force, and that God's election does not constrain our free will; and when someone is evil, He does not compel him to become good. God's election consists in this: that God predisposes us toward the good and grants us His grace. But to be saved and to act worthily of that election, or the opposite — this depends on our own mind and will.
Jn. 6:71. He said this about Judas, son of Simon Iscariot, for he was about to betray Him, being one of the twelve.
Consider also how fiercely the devil wages war. He was able to carry away one of the twelve apostles, who raised the dead, performed miracles, and heard the spiritual and life-giving teaching of the Lord. And where, then, is hope of salvation for us, when from our youth we have voluntarily enslaved ourselves to evil? Therefore the evangelist, as if in amazement, said: "being one of the twelve." "Although," he says, "he belonged to this holy company, yet by his evil will he proved unworthy of it." So easily is man inclined toward evil! Let the Manicheans not be deceived. We are not evil by nature. If that were the case, wickedness would constantly be at work in us. But Judas was not always evil; he was once even holy. Therefore evil both arises and grows from free will.