Romans 4:21 KJV
And being fully persuaded that, what he had promised, he was able also to perform.
Romans 4:19–23 KJV Chapter Context
19
And being not weak in faith, he considered not his own body now dead, when he was about an hundred years old, neither yet the deadness of Sara’s womb:
20
He staggered not at the promise of God through unbelief; but was strong in faith, giving glory to God;
21
And being fully persuaded that, what he had promised, he was able also to perform.
22
And therefore it was imputed to him for righteousness.
23
Now it was not written for his sake alone, that it was imputed to him;