John Gill 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
He had not only faith, a strong faith, but full assurance of faith:

that what he [God] had promised ;
though it was so long ago, and there were so many difficulties in the way;

he was able ,
being the Lord God Almighty,

to perform ;
so his faith rested upon the power of God, with whom nothing is impossible.