Albert Barnes Commentary Romans 4:21

"and being fully assured that what he had promised, he was able also to perform." — Romans 4:21 (ASV)

And being fully persuaded. Thoroughly or entirely convinced (Luke 1:1; Romans 14:5; 2 Timothy 4:5, 17).

He was able. . This was not the only time in which Abraham evinced this confidence. His faith was equally implicit and strong when he was commanded to sacrifice his promised son (Hebrews 11:19).