We look to the Scriptures and see that when Paul explains the doctrine of justification, he goes back to the Old Testament to Genesis 15. There the Scriptures say of Abraham, “He believed God, and it was counted to him for righteousness” (Gen. 15:6).
When Paul develops the doctrine of justification by faith alone, he is saying that when God counts somebody righteous on the basis of faith, it is not because He looks at them and sees that they are inherently righteous. Rather, they have been clothed by the imputation, or transfer, of the righteousness of Christ to that person by faith.
This is why we say that the single meritorious cause of our salvation is the transfer, or counting, of Jesus’ righteousness for me. Not only did He die to pay the penalty for my sins, but He lived a perfect life of obedience and fulfilled the law for those who put their trust in Him. This is what we’re talking about in imputation. That was the single, central, most important point of the sixteenth century Reformation.