Yet he [Abraham] did not waver through unbelief regarding the promise of God, but was strengthened in his faith and gave glory to God, being fully persuaded that God had power to do what he had promised. This is why “it was credited to him as righteousness.”Romans 4:20-22 NIV
In Genesis 15:5-6, we find a passage that was very important in Paul’s theology. God promised Abraham that he would have descendants as countless as the stars in the sky. Abraham believed what God had told him. And, because of that belief, God considered him as being righteous. Abraham’s righteousness did not come about because of anything that he did. But simply because he believed what God had told him. Even though, in his old age, it didn’t make any sense from a human perspective.
But Paul went beyond thinking that belief was simply an intellectual exercise. He said that Abraham was fully persuaded that God had the power to do what he had promised. Being fully persuaded, without wavering through unbelief, implies that he lived his life as though that promise was guaranteed. He lived believing that he would have descendants, even though he and Sarah were both old and childless.
As Paul goes on to say, the crediting of righteousness based on belief was not just for Abraham. But also for all who believe in God who raised Jesus from the dead. Our justification is not based on what we do. But on our faith in God and what he has done for us. Not just an intellectual acknowledgment, but a life-changing faith that leads us to follow Jesus as Lord.