What was the main purpose of Paul's letter to the Galatians?

Paul wrote Galatians to prove to Gentile believers in Galatia that they were not under the Mosaic Law but under grace. The letter confronts their desire to return to the weak and beggarly elements of Law, urging them not to go back into bondage after tasting grace.

Les explains that the whole thrust of this little book of Galatians was to prove to these Gentile believers up in Galatia that they were not under the Mosaic Law, they were under grace. He notes the Law was weak and beggarly, yet they thought they wanted to go back under it. Paul strongly opposed that, because once a believer has tasted of grace, it is a horror to return to the Law.

Galatians 4:8

Howbeit then, when ye knew not God, ye did service unto them which by nature are no gods.

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

Paul opposed it because the Law is weak and beggarly and brings bondage, while these Gentile believers were under grace, not the Mosaic Law. Having tasted grace and gained a position in Christ by believing the Gospel, they had no reason to return to the Law.