Why did Paul oppose the Galatians going back under the Law?

Les explains from Galatians 4:8-9 that Paul is talking about the Law when he asks how they could turn again to the weak and beggarly elements and desire again to be in bondage. The whole thrust of Galatians was to prove to the Gentile believers in Galatia that they were not under the Mosaic Law but under grace. Because the Law was weak and beggarly, yet they thought they wanted to go back under it, Paul said he could raise his voice against them. It was a horror to him that a believer would want to go back under the Law once they had tasted of grace. Les also connects this to Romans 8, where the righteousness or full purity of the Law as God first gave it was perfect, holy, and righteous, and that the righteousness of the Law might be fulfilled in us who are believers, who now have that position in Christ gained by believing the Gospel, having experienced salvation.