Cyril of Alexandria

Marvellous is the faith of the holy Apostles, fervent their manner of confession, most loveable and pre-eminent their understanding. For not like certain of the more ignorant, or like them who used to call the Word of the Saviour hard, did they rightly go back and fall, nor of lightness readily caught were they called to belief, but being fully assured beforehand and persuaded of a truth that their Instructor was full of life-giving Words, the Teacher of heavenly doctrines. …

Commentary on the Gospel of John, Book 4 ↗

Theophylact of Ohrid

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.

Commentary on John ↗

Augustine of Hippo

"And we have believed and have known." Not have known and believed, but "believed and known." For we believed in order to know; for if we wanted to know first, and then to believe, we should not be able either to know or to believe. What have we believed and known? "That Thou art Christ, the Son of God;" that is, that Thou art that very eternal life, and that Thou givest in Thy flesh and blood only that which Thou art.

Tractates on John 27 ↗

Thomas Aquinas

Then, from Peter's answer, we see the devotion of those who did not leave. For Peter — who loved the brethren, who guarded his friendships, and had a special affection for Christ — answered for the whole group, and said, Lord, to whom shall we go? You have the words of eternal life. Here he did three things. First, he extolled the greatness of Christ; secondly, he praised his teaching; and thirdly, he professed his faith. He extolled the greatness of Christ when he said, Lord, to whom shall we go? …

Commentary on John ↗

Bonaventure

"And we have believed and have come to know that you are the Christ, the Son of God." He made a similar confession in Matthew 16: "You are the Christ, the Son of the living God," "who have come into this world." He says "we believe," and then "we have come to know," because faith is the way into knowledge. "For unless you shall have believed, you shall not understand," Isaiah 7.

Commentary on John, Chapter 6 ↗