Ezekiel, son of man, the people living in the ruined cities of Israel are saying, "Abraham was just one man, and the LORD gave him this whole land of Israel. There are many of us, and so this land must be ours."
Abraham let himself be circumcised to show that he had been accepted because of his faith even before he was circumcised. This makes Abraham the father of all who are acceptable to God because of their faith, even though they are not circumcised.
God promised Abraham and his descendants that he would give them the world. This promise wasn't made because Abraham had obeyed a law, but because his faith in God made him acceptable.