Romans 4:22

Wherefore also it was reckoned unto him for righteousness.

Wherefore also ... gives the reason why Abraham's faith was reckoned unto him for righteousness, the reason being that Abraham truly believed, despite all human considerations to the contrary, the reality of which was not merely assumed upon Abraham's assertion that he believed, but which was proved by a long and exhaustive series of tests, beginning with the call to leave Ur and reaching the climax in the offering of Isaac. Though Abraham's obedience was not perfect, it was quite good enough to prove his faith. His obedience of every command, though not perfect, was practically sufficient. That obedience was the only ground upon which even God evaluated the faith of Abraham is implicit in the following:

And Abraham stretched forth his hand, and took the knife to slay his son. And the angel of the Lord called to him out of heaven, and said ... Lay not thine hand upon the lad; for now I know that thou fearest God, seeing thou hast not withheld thy son, thine only son from me (Genesis 22:11,12).

The inspired author James categorically stated that the justification of Abraham occurred "WHEN he brought up Isaac his son to the altar" (James 2:21).[16] Paul revealed that Abraham's faith justified him, without saying when; but James pinpointed the time. His faith justified him at that point in time when it was proved to be genuine, and that was in the instance of offering up Isaac. God said, "Now I know"; and this is fully equivalent to saying, "Before this, I did not know"!