For when you were slaves of sin, you were free in regard to righteousness (Rom 6:20). We were once outside the domain of righteousness, as it were. We were free from obeying its will. Our wills were united with sin and we willed what sin willed.
Now, however, in Christ, we are slaves of righteousness, and are free in regard to sin. We are free from obeying its demands – its power over us is made void and nothing. Our wills are united with God’s will. He fulfills His righteousness in us in the same manner as sin once fulfilled its unrighteousness in us.
The first slavery led to death and bondage. The second and last slavery leads to eternal life and freedom. The last slavery cannot be compared to the first. The first was without glory. The second comes with an eternal weight of glory.
Being one will with God is the same thing as being slaves of freedom.