For freedom Christ has set us free. Stand firm, then, and do not let yourselves be burdened again by a yoke of slavery. Christ has freed you from sin, from death, from the power of the devil, from the condemnation of the Law. He has freed you by His blood, shed on the cross, and by your Baptism, where He joined you to His death. Now that you are free, guard your freedom closely, and do not submit again to slavery.
Spiritual slavery comes in two forms.
There is slavery to the Law. Mixing our works with Christ's work. Trying to merit God's favor by commandment keeping. In the case of the Galatian churches, it was the idea that Gentile believers now had to be circumcized under the Law of Moses in order to be complete Christians. If you let yourselves be circumcized in obedience to the law of Moses, then Christ will be of no value to you.