Adam Clarke on Genesis 15:6

And he believed in the Lord; and he counted it to him for righteousness - This I conceive to be one of the most important passages in the whole Old Testament. It properly contains and specifies that doctrine of justification by faith which engrosses so considerable a share of the epistles of St. Paul, and at the foundation of which is the atonement made by the Son of God: And he (Abram) believed האמן heemin, he put faith) in Jehovah, ויחשבה לו vaiyachshebeita lo, and he counted it - the faith he put in Jehovah, to Him for righteousness, צדקה tsedakah, or justification; though there was no act in the case but that of the mind and heart, no work of any kind. Hence the doctrine of justification by faith, without any merit of works; for in this case there could be none - no works of Abram which could merit the salvation of the whole human race. It was the promise of God which he credited, and in the blessedness of which he became a partaker through faith. See at Gen 15:19 (note); see also on Romans 4 (note).