“Formerly, when you did not know God, you were enslaved to those that by nature are not gods.” (Gal. 4:8)
Here Paul is addressing the Gentiles in the Galatian churches. Formerly refers to before they learned of Christ and so before they obeyed the gospel preached by Paul. They knew a lot of gods or rather a lot about gods but they had no personal knowledge of the one true God, the creator of the universe.
Without the true God, there could only be false gods. Claims were made for gods who were not really gods at all but objects or mythical creatures that were proclaimed as gods. Of all those who wore the name god in the ancient world, only one called Himself, God. There is a major difference between stating you are God and someone stating it for you. Think about it, how can there be gods who cannot identify themselves but are only identified by their priests or followers?
Paul is back on the subject of slavery. He has talked about being a slave to the law but now he is talking about being a slave to false gods. How were they slaves and who enslaved them? It is obvious that since the gods had no power or voice that the slavery was voluntary. That was then, Paul is reminding them. But their current state is different. They have met the true God and His Spirit is among them. Knowing the true God and accepting Him as God freed them from their former enslavery.
To leave the gospel preached by Paul and given by God is to go backward. The gospel brought freedom and to change the gospel and add something else is to give up the freedom. The former state was one of ignorance and was somewhat excusable. Once they learned of God and obtained faith in Him, ignorance vanished. To leave the gospel then is to go back to a form of ignorance which is impossible because they are no longer ignorant.
Task for Today: You do not leave the gospel once you have taken it up in faith by ignorance. The only way for you to leave the gospel preached by Paul and other New Testament writers is to do so in the full knowledge that you want more than the original preaching of the Apostles offered. Do not add anything to the gospel. There is no Jesus plus. There is just Jesus. He is all you need and the gospel is all you need. Be aware of those who add to the gospel taught in the New Testament.