The Credit of Righteousness

Abraham is given a promise (a covenant) from the Lord that his descendants would become a great nation. And the Bible says that Abraham believed God and it was credited to him as righteousness. What's puzzling is that it doesn't sound as though Abraham and his wife believed that God would keep this promise; for they worked out an alternative plan, one that has led to centuries of conflict and struggle. Yet, righteousness was credited to Abraham because he believed. Let's take a look at this and try to discover how that could be.