After these things the word of the LORD came to Abram in a vision: “Fear not, Abram, I am your shield; your reward shall be very great.” But Abram said, “O Lord GOD, what will you give me, for I continue childless, and the heir of my house is Eliezer of Damascus?” And Abram said, “Behold, you have given me no offspring, and a member of my household will be my heir.” And behold, the word of the LORD came to him: “This man shall not be your heir; your very own son shall be your heir.” And he brought him outside and said, “Look toward heaven, and number the stars, if you are able to number them.” Then he said to him, “So shall your offspring be.” And he believed the LORD, and he counted it to him as righteousness.
So we read in Genesis 15 that Abra[ha]m was justified by faith.
Just like he had already been justified by faith before this event.
Thus we read in Hebrews 11:
By faith Abraham obeyed when he was called to go out to a place that he was to receive as an inheritance. And he went out, not knowing where he was going. By faith he went to live in the land of promise, as in a foreign land, living in tents with Isaac and Jacob, heirs with him of the same promise. For he was looking forward to the city that has foundations, whose designer and builder is God.
Not only do we have here a clear statement that Abraham had the faith “counted… to him as righteousness” before the events in Genesis 15, but he and his son and grandson also had the same afterward.
No surprise here. Paul himself describes Abraham’s faith not as a moment of conversion but as the belief that characterized his life:
Is this blessing then only for the circumcised, or also for the uncircumcised? We say that faith was counted to Abraham as righteousness. How then was it counted to him? Was it before or after he had been circumcised? It was not after, but before he was circumcised. He received the sign of circumcision as a seal of the righteousness that he had by faith while he was still uncircumcised. The purpose was to make him the father of all who believe without being circumcised, so that righteousness would be counted to them as well, and to make him the father of the circumcised who are not merely circumcised but who also walk in the footsteps of the faith that our father Abraham had before he was circumcised.
For the promise to Abraham and his offspring that he would be heir of the world did not come through the law but through the righteousness of faith. For if it is [exclusively] the adherents of the law who are to be the heirs, faith is null and the promise is void. For the law brings wrath, but where there is no law there is no transgression.
That is why it depends on faith, in order that the promise may rest on grace and be guaranteed to all his offspring—not only to the adherent of the law but also to the one who shares the faith of Abraham, who is the father of us all, as it is written, “I have made you the father of many nations”—in the presence of the God in whom he believed, who gives life to the dead and calls into existence the things that do not exist. In hope he believed against hope, that he should become the father of many nations, as he had been told, “So shall your offspring be.” He did not weaken in faith when he considered his own body, which was as good as dead (since he was about a hundred years old), or when he considered the barrenness of Sarah’s womb. No distrust made him waver concerning the promise of God, but he grew strong in his faith as he gave glory to God, fully convinced that God was able to do what he had promised. That is why his faith was “counted to him as righteousness.” But the words “it was counted to him” were not written for his sake alone, but for ours also. It will be counted to us who believe in him who raised from the dead Jesus our Lord, who was delivered up for our trespasses and raised for our justification.
The “footsteps of the faith” were the footsteps began at least when Abra[ha]m left Ur in response to God’s call (Genesis 12.1-3). Paul deliberately quotes from both Genesis 15 (“so shall your offspring be”) and and an event many years later in Genesis 17 (“I have made you the father of many nations”). Further, the “no distrust made him waver” does not seem to refer to only one event, but an ongoing trust. So too, “he grew strong in faith as he gave glory to God.”
So Abraham (I’m dropping the silly brackets) was justified by faith alone that night recorded in Genesis 15.6 and he was justified by faith alone before and after.