Reason does not see how great a matter this [Christian freedom] is (Gal 5:1); but when it is spiritually considered, it is enormous and infinite. No one can grasp with any thought what a great gift it is to have, in place of the Law, sin, death, and a wrathful God, the forgiveness of sins, righteousness, eternal life, and a God who is perpetually propitious and full of favor toward us. The papists and all hypocrites boast that they also have the forgiveness of sins, righteousness, a propitious God, etc.; and they also lay claim to freedom. They also promise this to others, but all these things are empty and uncertain. Truly these things are a service of corruption. In temptation they vanish in a moment, because they depend on human works and satisfactions, not on the Word of God, nor on Christ. Therefore it is impossible for them to know what freedom from sin, etc., really is.
By contrast, our freedom has Christ as its foundation, who is the eternal High Priest, who is at the right hand of God and intercedes for us. Therefore the forgiveness of sins, righteousness, and life, and freedom that we have through Him are certain, completed, and eternal, provided that we believe this. If we cling firmly to Christ by faith and stand firm in the freedom with which He has made us free, we shall have those unspeakable gifts. But if we become secure and drowsy, we shall lose them. It is not in vain that Paul commands us to be vigilant and to stand fast, because he knows that the devil is busily engaged in trying to rob us of this freedom that cost Christ so much, and to entangle us again in the yoke of slavery through his ministers.
--Martin Luther, Commentary on Galatians