In Romans, Paul was writing to believers in Rome.
He wrote:
If you are a slave to sin, you get eternal death.
If you are a slave to obedience, you get righteousness.
(Romans 6:16)
P.S. If you slip up, thereby failing to be righteous,
you can always confess/repent of your sin,
and the Lord will cleanse you from all unrighteousness
(1 John 1:9) via Jesus' precious blood.
John was writing to believers, right?
But thanks be to God that, though you used to be slaves to sin, you have come to obey from your heart the pattern of teaching that has now claimed your allegiance.
18 You have been set free from sin and have become slaves to righteousness. Rom6:17&18
We come to Christ as slaves to sin/sin dominates our lives, and we are called to cross over into being slaves of righteousness, leading to holiness, but this crossing over from one state to the other takes time.
But if, in seeking to be justified in Christ, we Jews find ourselves also among the sinners, doesn’t that mean that Christ promotes sin? Absolutely not!
18 If I rebuild what I destroyed, then I really would be a law-breaker. Gal2:17&18
In the above Paul is speaking of a justification that is not instantaneous, it is one which takes time. You are immediately justified at the point of conversion by faith in Christ, that places you in a justified state, so what justification is Paul speaking of in the above? He is speaking of a new convert, in respect of crossing over, from being a slave of sin when they come to Christ, into a slave of righteousness
leading to holiness. They seek this justification, not by works of the law/observing the law/striving not to commit sin, but by faith in Christ. They trust in Christ to cross over from one state to the other. This is not a licence to sit back on your backside and do nothing of course, for the new convert has been born again.
Paul states that while this crossing over from one state to the other is taking place, the convert will be found to be a sinner, or it is evident they are a sinner. Why is this? Because some sin you can immediately cease at the point of conversion, for it has not tied itself so tightly around you, that you are its slave, however, other sin you may have been a slave to for years, if not decades, and this sin can take longer to see defeated.
Paul then asks the question: Does Christ promote sin? Why would he ask that? Imagine a new convert, who is crossing over from one state to the other. He joins a church, and heartily joins in the service. What would someone who did not understand the message think? ‘’’If this person is a Christian Christ must promote sin, for here he is joining us in worship with glaring sin in his life’’’ However, what the person with such thoughts fails to understand is, the new convert hates his sin, for he has been born again, the last thing he wants is it to stay there, but he is still entitled to rejoice in what Christ has done for him, despite his imperfections or he must go around in sackcloth and ashes until he is as pure as the driven snow.
Immediately after Paul asks the question he answers it: ‘’Absolutely not, if I rebuild what I destroyed I prove I am a lawbreaker(NIV 1984 edition) What has Paul sought so hard to destroy? Righteousness of obeying the law. Therefore, if he returned to seeking such justification by obeying the law/striving not to commit sin, he would fail, and simply prove he was a lawbreaker/sinner.
Of course, if Jesus only died for past sins, the text could not be followed.