What was the whole point of the book of Galatians?
The whole point of Galatians was to prove to Gentile believers in Galatia that they were not under the Mosaic Law but under grace. Paul argued the Law was weak and beggarly, and he was astonished that believers who had tasted grace would want to return to bondage under the Law.
Les explains that in Galatians 4:8-9, Paul asks how they could turn again to the weak and beggarly elements, and Les says the subject is the Law. That was the whole thrust of this little book of Galatians: to prove to these Gentile believers up in Galatia that they were not under the Mosaic Law, they were under grace. Because the Law was weak and beggarly, yet they thought they wanted to go back under the Law, and Paul just said he could raise his voice against them. Les asks, horror of horrors, why would a believer want to go back under the Law once we have tasted of grace? He then connects this to Romans 8, where the righteousness or full purity of the Law as God first gave it was perfect, holy, and righteous, so that it might be fulfilled in us who are believers and who now have that position in Christ, gained by believing the Gospel.