What a beautiful passage! It is among the most helpful in all the Bible explaining God's reasons for designing faith not works as his prerequisite for salvation. One key is in Romans 4:4, God doesn't like to be someone's debtor or to be obligated to us.
Abraham believed God and it was credited to him as righteousness. He did not need prerequisites like circumcision. He did not conform to the Law as a basis for his standing before God. It was given by grace, not obligation because he believed the one who promised was able to keep his promise (Romans 4:13). He could call things that are not as though they are. He gives life to the dead. I want that kind of faith. Lord, I believe, help my unbelief!