John Chrysostom

Behold another miracle not less than the former. "And beholding the boldness of Peter and John," etc. "And they took knowledge of them that they had been with Jesus." Not without a meaning has the Evangelist set down this passage; but in saying, "they recognized them that they had been with Jesus," he means, in His Passion: for only these were with Him at that time, and then indeed they had seen them humble, dejected: and this it was that most surprised them: the greatness of the change. …

Homily on Acts 10 ↗

Theophylact of Ohrid

It is possible to be both unlettered and not simple, and simple yet not unlettered; but here both coincided (that is, both lack of learning and simplicity). Therefore the Jews were amazed when Peter and John spoke and orated. From where then did they obtain this knowledge that Peter and John were such? From the words of Peter and John themselves. For Luke does not simply say "they knew them," but "they recognized them, that they had been with Jesus" during His suffering. At that time they alone were with Him. …

Commentary on Acts ↗

Bede

But seeing the boldness of Peter and John, and realizing that they were unlettered and ordinary men, they marveled. "Unlettered" is said, not because they were ignorant of letters, but because they were not skilled in the art of grammar. For in Greek, it is more clearly, for this word, "ἀγράμματοι," that is, unlettered, and "idiotae" are called unskilled. Finally, in the Epistle to the Corinthians, where it is written: "And if unskilled in speech, yet not in knowledge" (2 Cor. XI), for "unskilled" in Greek it is "ἰδιώτης."

Retractions on Acts ↗