"Abram believed the Lord, and he credited it to him as righteousness." God promised Abram that he would have a son and heir and that his offspring would be as numerous as the stars in the sky. This promise finds its fulfilment not only in the nation of Israel but also in all people who have become heirs of his kingdom through faith in Christ.
Abram simply took God at His word and relied on His promise and was justified by faith. This is the first mention of faith in the Bible and Paul refers to this passage in establishing the New Testament principle of justification by faith in Romans 4.
God made his promise clear to Abram by instructing him to make a sacrifice. Abram divided the sacrificed animals (except the birds) into two parts and laid each half on the ground opposite each other as was the custom of the day in making a covenant. The idea was that in providing a passage between the divided animals both parties to the covenant would walk between them and
around them usually in a figure of eight pattern. The agreement was that if either party should break the covenant, they would be cut in half as the animals were. This covenant still stands open: Jesus is the Lamb slain before the throne and we can all ratify the covenant for ourselves by believing and trusting in Him.
(Men's Ministry Leadership Member)