Rom 4:20-21, 20 He did not waver at the promise of God through unbelief, but was strengthened in faith, giving glory to God, 21 and being fully convinced that what He had promised He was also able to perform.
The Apostle shows the power and the excellence of that faith of Abraham to which he ascribes justification. “Abraham was a man of great faith in God, he is known as the father of faith, but that is not a compliment to Abraham. He merely caught a glimpse of God’s incredible ability, His impeccable integrity, and His wonderful faithfulness to keep every promise He makes. His faith gave “glory” to a faithful God.” (Ray Comfort).
Abraham was fully persuaded and certain that God not only could, but would do what He promised. God indeed is able to make good His own word in spite of all obstacles working against it. It demonstrates the faithfulness and the reliability of our God. The unshaken faith holding on to the promises of God is strengthened by God. This is the faith that pleases God (Heb 11:6) for it is the gift of God preserved by God that gives God all the glory.