"Faith always looks at problems in the light of the promises ... He knew that God could keep His promises (because of His power) and He knew that He would do so (because of His faithfulness). He was fully persuaded that God had power to do what He had promised. This is why, Paul adds, namely because He belived God's promise; "It was credited to him as righteousness".
John R W Stott - "BST Series - The Message of Romans" - IVP, Leicester (1994) - (pp134-135).