Paul here was referring to Genesis 15:6, as he also did in Romans 4:3, and as James did in James 2:23. Thus, as he repeatedly stresses, the doctrine of justification by faith in Christ, accompanied by the corollary truth of having Christ's perfect righteousness imputed to us while He is made sin for us and bears our penalty, is not merely a divine afterthought following the failure of Israel under the law. It was the very means by which Abraham, the patriarchal father of Israel, was saved and which continues to apply today.

Source:

KJV Defenders Study Bible, by Dr. Henry Morris, Ph.D.
Publisher: Thomas Nelson
Language: English
DEF 10 ISBN 0-529-10444-x
DEF 10-1 ISBN 0-529-10445-8