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.

Abraham [to use his later name] has, at this point in the story, just taken up arms against a great military empire.  He has reason to be afraid and it makes sense that God would give Abraham assurance at this time.

Of course, once assurance against immediate threats is mentioned, Abraham thinks of his angst over some other issues.  So God promises to provide him with a child and a great number of descendants eventually.

God regards Abraham’s trust in Him that he will keep his promise to be righteousness.

You can hear the definition of faith given by the author of Hebrews in this paragraph:

Now faith is the assurance of things hoped for, the conviction of things not seen. For by it the people of old received their commendation. By faith we understand that the universe was created by the word of God, so that what is seen was not made out of things that are visible. By faith Abel offered to God a more acceptable sacrifice than Cain, through which he was commended as righteous, God commending him by accepting his gifts. And through his faith, though he died, he still speaks. By faith Enoch was taken up so that he should not see death, and he was not found, because God had taken him. Now before he was taken he was commended as having pleased God. And without faith it is impossible to please him, for whoever would draw near to God must believe that he exists and that he rewards those who seek him.

Abraham couldn’t see his future descendants, but he trusted God to keep his promise.  He believed that God “rewards those who seek him,” and God indeed promised, “I am your shield; your reward shall be very great.”

Paul too saw Abraham’s faith and God’s regarding it as righteousness to be a proof of the Christian faith, and the antithesis of boasting:

What then shall we say was gained by Abraham, our forefather according to the flesh? For if Abraham was justified by works, he has something to boast about, but not before God. For what does the Scripture say? “Abraham believed God, and it was counted to him as righteousness.” Now to the one who works, his wages are not counted as a gift but as his due. And to the one who does not work but believes in him who justifies the ungodly, his faith is counted as righteousness, just as David also speaks of the blessing of the one to whom God counts righteousness apart from works:

“Blessed are those whose lawless deeds are forgiven,
and whose sins are covered;
blessed is the man against whom the Lord will not count his sin.”

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.

Paul goes on to translate this in terms of how we are assured by God through the Gospel of the death and resurrection of Jesus: When we say “Jesus is Lord” and believe that he was raised from the dead to that position, we are claiming that he is our great shield and that our reward will be very great.

For Moses writes about the righteousness of the law, that the person who does the commandments shall live by them. And the righteousness of  faith says, “Do not say in your heart, ‘Who will ascend into heaven?’” (that is, to bring Christ down) or “‘Who will descend into the abyss?’” (that is, to bring Christ up from the dead). But what does it say? “The word is near you, in your mouth and in your heart” (that is, the word of faith that we proclaim); because, if you confess with your mouth that Jesus is Lord and believe in your heart that God raised him from the dead, you will be saved. For with the heart one believes and is justified, and with the mouth one confesses and is saved. For the Scripture says, “Everyone who believes in him will not be put to shame.” For there is no distinction between Jew and Greek; for the same Lord is Lord of all, bestowing his riches on all who call on him. For “everyone who calls on the name of the Lord will be saved.”

God regards that trust as our righteousness.  We don’t need to seek another.