What does Galatians 4:8 mean?

Paul is writing to a group of Christians in Galatia who are beginning to be influenced by false teachers to start following the law of Moses in order to be acceptable to God (Galatians 2:4). Using several different approaches, Paul has made a case as to why it does not make sense to seek salvation through the law, when Christ came at just the right time to save them, in His grace, by faith.

Paul has also reminded the Galatians of all that comes with being saved by faith in Christ, as opposed to struggling to earn merit by trying to follow the law. They have been adopted by God as His very children. They have a share in the inheritance of Jesus Christ. God's own Spirit has come to live in their hearts so that they can call out to God as their own Abba, a word which means "father."

Now Paul takes the Galatians back to their spiritual condition before they trusted in Christ. Then, they were not even followers of the law of Moses. They were slaves to other figures, like Zeus and Hermes, who were not even true gods, at all. Their false religion held them captive to false hopes of being saved or favored by false gods. Paul seems to be saying that he knows the Galatians don't want to go back to worshiping those gods.

The following verse follows from this assumption. He will ask why, then, they would want to be slaves again to the worldly principles that now come with following the law.