Вполне

21 И бу́дучи вполне уве́рен,

21 And being вполне [certain, convinced, i am sure, persuaded, sure],

что Он силе́н и испо́лнить обе́щанное.

[what, that, why] He [mighty, powerful, strong] and [execute, fulfill, perform] [promised, the promised].

And being fully persuaded that,
what he had promised,
he was able also to perform.
Romans 4:21 KJV