Moo’s note on Romans 6:16 is a good reminder that we are always in a state of moral progress or moral decay:
Either one is a slave of sin or a slave of “obedience.” Paul makes clear in this “either … or” that there is no “possibility of neutrality” (Käsemann). One is never “free” from a master, and those non-Christians who think that they are “free” are under an illusion created and sustained by Satan. The choice with which people are faced is not “Should I retain my freedom or give it up and submit to God?” but “Should I serve sin or should I serve God?” But Paul is writing to Christians, and we, too, must not forget that to sin is to submit ourselves as slaves to sin.
The Letter to the Romans. (NICNT, 2nd ed.) p. 425
For a list of all of my posts on Douglas Moo’s commentary on Romans, see here.