What was the Law's weakness?
The Law's weakness was that it was fleshly, weak, and beggarly, so it could not make anyone perfect or right with God. Salvation began in the Spirit by faith, and the works of the Law, which are of the flesh, cannot complete that work. Faith, not Law-works, is what God recognizes.
Les explains that when Paul asks if the Galatians are now made perfect by the flesh, the flesh is a reference to the Law. The Law was fleshly, weak, and beggarly. They had begun in the Spirit—the Holy Spirit generated salvation because of their faith—yet they were being told they could be made right with God by the flesh, that is, by the works of the Law. But Scripture says it is by faith and not the works of the Law. Faith is taking God at His Word: God said it, we believe it, and God recognizes it, and on that basis He moves in and does everything that needs to be done. Hearing involves no work; faith comes by hearing, not by Law-works. Thus the Law's weakness is its fleshly inability to perfect what the Spirit began.