All believers are justified the same way, namely by faith. Abraham (pre-law), Moses (law), and Paul (post-law) were all justified by faith. Righteousness is not earned through keeping any law. It is a gift received by faith. If it came through the law, faith would be meaningless (Romans 4.14). The promise made to Abraham would be meaningless because he lived before the giving of the law. The law brings about wrath. It informs us of the fact that we have overstepped God’s boundaries. We have transgressed against a holy God. Where there is no law, there is no transgression (Romans 4.15).
Abraham is the father of ALL who believe (Romans 4.11). He is the father of both Gentile and Jewish believers (Romans 4.12). It is in this sense that he is the father of us all (Romans 4.16).
Therefore it is of faith that it might be according to grace, so that the promise might be sure to all the seed, not only to those who are of the law, but also to those who are of the faith of Abraham, who is the father of us all (as it is written, “I have made you a father of many nations”) in the presence of Him whom he believed—God, who gives life to the dead and calls those things which do not exist as though they did; who, contrary to hope, in hope believed, so that he became the father of many nations, according to what was spoken, “So shall your descendants be.” And not being weak in faith, he did not consider his own body, already dead (since he was about a hundred years old), and the deadness of Sarah’s womb. He did not waver at the promise of God through unbelief, but was strengthened in faith, giving glory to God, and being fully convinced that what He had promised He was also able to perform. And therefore “it was accounted to him for righteousness” (Romans 4.16-22).
Abraham believed God. This lead to strength, hope, and confidence in his life. Abraham’s faith never weakened even thought his body and Sarah’s womb were as good as dead (Romans 4.19). He did not waver, but held fast to the promise and character of God. Faith is being fully convinced that what God has promised, He is also able to perform (Romans 4.21).
Do not look at YOUR faith as weak or strong. We ourselves may be weak, but our strength and hope come from the object of our faith. That object is the promise and character of God. Faith is human trust in divine promises. We don’t marvel at Abraham’s strength but at God’s strength. God strengthens us because we trust in Him. It matters little if our faith is feeble or confident. It matters a great deal when we ask about the promise and character of the One in whom we trust.