What was the flip side of Israel's ignorance in Acts 3?
The flip side was that although Israel acted in ignorance, God fulfilled what He had foretold by all the prophets—that Christ must suffer. Peter declares the crucifixion happened in ignorance, yet it also fulfilled prophecy, which had to come to pass to the last jot and tittle.
Les explains that in Acts 3:17 Peter acknowledges Israel crucified the Lord through ignorance, as did their rulers. But the flip side is Acts 3:18: those things God before had shown by the mouth of all His prophets, that Christ should suffer, He hath fulfilled. Anytime prophecy is written, it must be fulfilled to the last jot and tittle, or it would make God a liar. Everything written prophetically has to come to pass, and everything still future will happen. Peter is saying Christ had to die because prophecy had foretold it. This follows the same format as Acts 2:38, leading to verse 19: Repent therefore and be converted—turn your direction, change your thinking.
But those things, which God before had shewed by the mouth of all his prophets, that Christ should suffer, he hath so fulfilled.