“And he believed in the Lord, and He accounted it to him for righteousness” (Genesis 15:6, NKJV).
God has always been full of mercy and grace, even in the Old Testament when the works-based Law of Moses was in effect. When God told Abram he would be the father of many nations despite his advanced age, it came to pass because Abram believed God. His faith in God’s promises, not his actions, was what made him righteous.