Why Then the Law? How the Promise to Abraham & the Law of Moses Fit Together (Genesis 15; Galatians 3:6-29)

Paul shows clearly that Abraham was declared righteous by faith, not works in Genesis 15. If Abraham was saved by faith not works, and we are saved by faith not works, then why was there a Mosaic Law Covenant at all? "Why then the law?" is the question Paul asks. The answer may be more relevant than you think.