Romans 4:21

and being fully convinced that what He had promised He was also able to perform.

Surrounding context

4:20 He did not waver at the promise of God through unbelief, but was strengthened in faith, giving glory to God,

4:21 and being fully convinced that what He had promised He was also able to perform.

4:22 And therefore "it was accounted to him for righteousness."

Translation and sources