John Gill Commentary Genesis 15:6

"And he believed in Jehovah; and he reckoned it to him for righteousness." — Genesis 15:6 (ASV)

And he believed in the Lord
The Targums of Onkelos and Jonathan are,

``in the Word of the Lord;'' in the essential Word of the Lord, in Christ the Lord his righteousness; he believed in the promise of God, that he should have a seed, and a very numerous one; he believed that the Messiah would spring from his seed; he believed in him as his Saviour and Redeemer; he believed in him for righteousness, and he believed in his righteousness as justifying him before God:

and he counted it to him for righteousness ;
not the act of his faith, but the object of it; and not the promise he believed, but what was promised, and his faith received, even Christ and his righteousness this was imputed to him without works, and while he was an uncircumcised person, for the proof of which the apostle produces this passage, (Romans 4:3Romans 4:10Romans 4:23Romans 4:24) ;

wherefore this is not to be understood of any action of his being esteemed and accounted a righteous one, and he pronounced and acknowledged a righteous person on account of it; for Abram was not justified before God by his own works, but by the righteousness of faith, as all that believe are, that is, by the righteousness of Christ revealed to faith, and received by it: what is imputed is without a man, and the imputation of it depends upon the will of another; such the righteousness of Christ without works imputed by God the Father. This is the first time we read of believing, and as early do we hear of imputed righteousness.