John Chrysostom
Because he had sharply rebuked them and shown that the Crucified One had risen, he again softens his speech. He gives them the opportunity to repent and says:
"Yet now, brethren, I know that you did it in ignorance, as did also your rulers." "In ignorance" is one defense. The other is, "as did also your rulers."
What Joseph said to his brothers, "God sent me before you" (Gen 45:5), or rather what Peter himself had earlier said briefly, he now says more fully.
Earlier he said, "Him, being delivered by the determined counsel and foreknowledge of God, you took" (Acts 2:23). Then the apostle at once calmed their frightened souls.
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He called them brethren and said, "Men, brethren." There he said nothing about himself, but only about Christ: "Let all the house of Israel know assuredly" (Acts 2:36).
Here, however, he also offers exhortation. There he waited for the Jews to speak. Here, after the apostles had already done so much, he knew they were more capable of receiving his words.
Without doubt, even what was said above did not arise from ignorance. They had asked for a robber. They did not receive Him whom Pilate had judged should be released.
They even wished to kill Him. What ignorance was there in this? Yet despite all this, he gives them opportunity to deny and repent of what they had done.
He even presents a plausible defense for them. He says: You knew that you killed an innocent man. But perhaps you did not know that you killed the Author of life.
In this way he excuses not only them from the crimes, but also the chief authors of the evil. Had he turned his speech into accusation, he would have made them more stubborn.
When someone who has done something terrible is accused, he becomes more obstinate while trying to justify himself. So Peter no longer says, "You crucified," or, "You killed."
He says, "you did it," and thereby leads them toward forgiveness. If those acted in ignorance, much more did these. If forgiveness is granted to those, it is granted much more readily to these.