This is the freedom and the slavery of which Paul speaks in Rom. 6:20, 22: “When you were slaves of sin, you were free in regard to righteousness. But now that you have been set free from sin, you have become slaves of God.” But let us set this up in a diagram:
Freedom from righteousness » Service of sin
Service of righteousness » Freedom from sin
For he who is free from sin has become a slave of righteousness; but he who is the slave of sin is free from righteousness, and vice versa. . . . “Christ,” he says, “has made us free with this freedom.” It is a spiritual freedom, one to be preserved in the spirit. It is not that heathen kind, which even the pagan Persius knew was not enough. It is freedom from the Law, but in a way contrary to what usually takes place among men. For it is human freedom when laws are changed without effecting any change in men, but it is Christian freedom when men are changed without changing the Law. Consequently, the same Law that was formerly hateful to the free will now becomes delightful, since love is poured into our hearts through the Holy Spirit (Rom. 5:5). In this freedom, he teaches us, we must stand strongly and steadfastly, because Christ, who fulfills the Law and overcomes sin for us, sends the spirit of love into the hearts of those who believe in Him. This makes them righteous and lovers of the Law, not because of their own works but freely because it is freely bestowed by Christ.
(Blessed Martin Luther, commentary on Galatians v. ; Luther’s Works, vol. 27, ed. Jaroslav Pelikan; CPH: St. Louis, 1964, 1992; pp. 325-326)