At the 2005 Annual meeting of the Evangelical Theological Society in Valley Forge, PA, famed Old Testament scholar Dr. Walter Kaiser explained Gen 15:6. He said that it teaches that Abraham did not merely believe in God generally, but that he specifically believed God’s promise that the Messiah would come through his (Abraham’s) body (Message 79; Time 39:00ff.). Kaiser indicated that anyone who ever has or ever will be saved will only do so by faith in Christ. No one has been or ever will be justified by works.
Kaiser also indicated that people who’ve never heard of Christ still must believe in Him to be born again just as people in OT times had to believe in the coming Messiah to be saved.