Then he believed in the Lord; and He reckoned it to him as righteousness.At the beginning of Chapter 15, Abram had just defeated the eastern kings, and it really hadn't left him any better off. Even though God had promised him a great nation, he still didn't have any land, nor any children. His nephew, Lot was who he hoped might succeed him was living outside of the land of promise in Sodom. At this point, his most likely heir is his servant Eliezer (it was common in Mesopotamia to adopt a servant as a male heir).
God then addresses Abram's disappointment: “Do not fear, Abram, I am a shield to you; Your reward shall be very great.” At this, Abram decided to share with God HIS sense of disappointment. This didn't cause an angry response from God, instead God reaffirmed that he would father a son, and he would have descendants that were as countless as the stars in the sky.
This satisfied Abram: "Then he believed in the Lord; and He reckoned it to him as righteousness." After this, he kept believing; he kept relying on God.
One thing that grabbed me when studying Genesis 15 was v.6. Three words stick out to me: Believed...Reckoned...as Righteousness. Paul quotes these as an illustration of faith in contrast to works in Romans 4:3,9,20-22 and Galatians 3:6 -- James also in 2:23. Abraham was justified by faith. Genesis 15:6 is a crystal clear statement about justification. Abram was a man of faith -- works are never the grounds of justification. Good works are a byproduct of faithfulness.
The word "justify" is translated from the Greek verb δικαιόω or 'dikaioo' -- it can be a legal term used to indicate a favorable verdict in a trial. Picture a courtroom setting with God presiding as the Judge, judging everyone on the faithfulness of that person to the law. At the beginning of Romans, Paul makes it clear that nobody can withstand God's judgement (Romans 3:9-20). Think about it this way though -- the law wasn't created to justify sinners -- it was made to EXPOSE their sin. As a solution to this problem God sent his Son to die for US -- for our sins in our place. So, when we believe in Jesus, God attributes HIS righteousness to us. God is not only the righteous Judge, he is the one who declares us righteous as our justifier (Romans 3:26)
"We are not made righteous by doing righteous deeds; but when we have been made righteous, we do righteous deeds." -- Martin Luther