Therefore, is the law against the promise from God? May it never come to be! For if the giving of law, one having the inherent ability to give life, certainly out from the law would be the righteousness (Galatians 3:21).
The Mosaic Law did not replace the covenant of promise. When God brought Israel out of Egypt to Mount Sinai, He instructed them to obey His voice and guard His covenant (Exodus 19:5). This covenant is the promise He gave to Abraham concerning land (Genesis 15:18–21). Israel’s response was to pledge to God that if He keeps on telling them what to do, they will do it, rejecting living by the covenant of promise (Exodus 19:8). Once a covenant is made, it cannot be annulled or added to (Galatians 3:15). Therefore, the law does not negate the promise.
If there were a law by which righteousness could come, then the covenant given to Abraham would have been based on that law. However, through the flesh no man can be justified before God (Romans 3:20). Therefore, the Mosaic law was not given as a means for men to obtain righteousness, but as a school master to guard Israel until faith in Christ came, so that the promise out from faith in Jesus might be given to all who believe (Galatians 3:22–23).