17 And now, brethren, I wot that through ignorance ye did it, as did also your rulers.
18 But those things, which God before had shewed by the mouth of all his prophets, that Christ should suffer, he hath so fulfilled.
19 Repent ye therefore, and be converted, that your sins may be blotted out, when the times of refreshing shall come from the presence of the Lord.
20 And he shall send Jesus Christ, which before was preached unto you:
Peter is telling national Israel that because they did it in ignorance, recall Jesus's prayer at the cross for God to forgive Israel because they know not what they do, if they are willing to repent and be baptized, even this sin could be blotted out.
It was an offer to national Israel.