Each profession has its own diagnostic tools. In my line of work we have thermometers, stethoscopes, x ray machines and many other ways of finding problems in the human body.
In this seemingly technical passage, Paul is using a legal example to explain to his readers the limitations of the Law. If you recall, the Galatian people had started slipping back into wanting to observe the Law, rather than depending on simple faith in Christ. Ok, Paul says, let me explain. If people make a covenant, an agreement, no one can set it aside or add to it. In the same way, God made a covenant with Abraham, a promise that Christ would come. 430 years later, the Law was added, and it did not take away the promise of Christ.
So, Paul says, what then is the purpose of the Law? (v. 19). Does following the Law impart righteousness? Well no, Paul says. It hasn’t done that yet so we can stop holding our breath on that one.
This is where the diagnostic tools come in. Paul says that the Law shows that we have a problem inside. We can’t be made right with God by following rules, but only by faith in Christ. The Law is our stethoscope, our X ray, the funny noise our car is making as we drive it to the shop. Now that we know we have a problem, thank God that we can throw ourselves on the mercy of Christ. May we thank Him today for the gift of faith and for His sacrifice.