Through your offspring all peoples on earth will be blessed.

Acts 3:17-26
For Moses said, ‘The Lord your God will raise up for you a prophet like me from among your own people; you must listen to everything he tells you. Anyone who does not listen to him will be completely cut off from their people.’
“Indeed, beginning with Samuel, all the prophets who have spoken have foretold these days.
God said to Abraham, ‘Through your offspring all peoples on earth will be blessed.’ When God raised up his servant, he sent him first to you to bless you by turning each of you from your wicked ways.”
Why?
Peter
not only told how to repent and return to the Lord and spoke Moses, Samuel and Abraham many prophets had foretold this day and Jesus comes from them. And Jesus is plan of the Lord.
Only follow Jesus Christ and trust in Him is the way of blessing.
For the Holy Spirit is with Peter so he could say so much words to the crows in the Temple. He detail explain all the story of Jesus Christ and with authority to teach the crowd.
How?
Wait for the Holy Spirit and God's blessing comes over to those who follow God's word.