What does Galatians 4:9 mean?

Paul has made a bold and compelling case that the arrival of Christ created the opportunity for his Galatian readers to be made right before God—"justified"—by faith alone. They need not—they should not—begin to follow the law of Moses in hopes of being justified, when God had already given that to them through Christ.

Now Paul compares following the law to the religious beliefs these Greek Christians held before they heard of Jesus. Back then, Paul wrote in the previous verse, they were slaves to false gods like Zeus and Hermes, who were not even real beings, at all. They worshiped these gods in hopes of being favored, becoming slaves to imaginary masters.

Paul now suggests that legalistic obedience to the law of Moses, after Christ has come, brings with it the same slavery as worshiping false gods like Zeus. They will be making themselves slaves to the basic principles of the world again, just as they did before they knew Christ. What are these "basic principles"? In part, they involve the idea that human beings can somehow prove themselves worthy to the gods–or to God Himself—by leading their lives in a specific way. The attempt to "earn" one's salvation is a common thread in all man-made religions.

Paul has already shown that this cannot be done. Humans cannot earn the favor of non-existent gods. Nor can they earn the favor of the one, true God, because they will never be able to follow His law perfectly. The law showed that we are all slaves to our own sinfulness; we must be rescued.

Now that his readers have come to know God, and God has come to know them, through their faith in Christ, why return to that slavery? There simply is no good reason.