Why does Peter refer to the covenant which God made to Abraham?
Acts 3:25 (ESV)
25You are the sons of the prophets and of the covenant that God made with your fathers, saying to Abraham, ‘And in your offspring shall all the families of the earth be blessed.’
Peter is making the claim that Jesus is the offspring of Abraham through whom blessing comes. Jesus is thus the one of whom God spoke when he made his great promise to Abraham.
25 You are the sons of the prophets and of the covenant that God made with your fathers, saying to Abraham, ‘And in your offspring shall all the families of the earth be blessed.’