It was because Abraham confidently believed God’s promise to give him many descendants, when there was no physical basis for his hoping that this would happen, that God erased the record of his sins.

Romans 4:17b-22

God guaranteed that he would give Abraham many descendants. Abraham confidently believed that God would do that, 18 even though there was no physical reason for him to hope that he would have descendants, because he and his wife were too old to bear children. But God is the one who causes dead people to live again, and who talks about things that do not yet exist as already existing. God said to Abraham, “You will have so many descendants that they will be as impossible to count as the stars.” And Abraham believed that, and he believed that he would become the ancestor of many ethnic groups. 19 He did not doubt that God would do what he promised, even though he knew that his body was already as incapable of begetting children as if he were dead [MET] because he was about 100 years old. And even though he realized that Sarah had never been able to become pregnant [IDM], 20 he did not doubt at all that God would do what he had promised. Instead, he trusted in God more/very strongly, and he thanked God for what God was going to do. 21 He was also convinced {very sure} that the thing that God promised, God was able to do. 22 And that is the reason that the record of his sins was erased {God erased the record of his sins}.