What does it mean that Abraham was fully persuaded in Romans 4:21?
Being fully persuaded means Abraham had not an ounce of doubt that God would still do what He promised. He was strong in faith, giving glory to God, and trusted that God was able to perform what He had promised. His believing, plus nothing, was imputed to him as righteousness.
Les explains that in Romans 4:20-21 Abraham staggered not at the promise of God through unbelief, but was strong in faith, giving glory to God. Being fully persuaded meant there was not an ounce of doubt in Abraham's mind that God was still going to do what He said. Because he was fully persuaded of what God had promised, God was able also to perform it. His believing plus nothing was imputed—a bookkeeping term in the Greek—placed to his account on the ledgers of heaven, and he is now a righteous man.
And being fully persuaded that, what he had promised, he was able also to perform.