That separation between the two slaves is not scriptural. If you only read the part about two slaves, it might appear so to you, but let us read on:
18 Being then freed from sin, we have been made servants of justice. 19 I speak an human thing, because of the infirmity of your flesh. For as you have yielded your members to serve uncleanness and iniquity, unto iniquity; so now yield your members to serve justice, unto sanctification. 20 For when you were the servants of sin, you were free men to justice. 21 What fruit therefore had you then in those things, of which you are now ashamed? For the end of them is death. 22 But now being made free from sin, and become servants to God, you have your fruit unto sanctification, and the end life everlasting. 23 For the wages of sin is death. But the grace of God, life everlasting, in Christ Jesus our Lord.[...]
6 But now we are loosed from the law of death, wherein we were detained; so that we should serve in newness of spirit, and not in the oldness of the letter. 7 What shall we say, then? Is the law sin? God forbid. But I do not know sin, but by the law; for I had not known concupiscence, if the law did not say: Thou shalt not covet. 8 But sin taking occasion by the commandment, wrought in me all manner of concupiscence. For without the law sin was dead. 9 And I lived some time without the law. But when the commandment came, sin revived, 10 And I died. And the commandment that was ordained to life, the same was found to be unto death to me 11 For sin, taking occasion by the commandment, seduced me, and by it killed me. 12 Wherefore the law indeed is holy, and the commandment holy, and just, and good. 13 Was that then which is good, made death unto me? God forbid. But sin, that it may appear sin, by that which is good, wrought death in me; that sin, by the commandment, might become sinful above measure. 14 For we know that the law is spiritual; but I am carnal, sold under sin. 15 For that which I work, I understand not. For I do not that good which I will; but the evil which I hate, that I do.
(Romans 6-7)
Compare:
43 It is sown in dishonour, it shall rise in glory. It is sown in weakness, it shall rise in power. 44 It is sown a natural body, it shall rise a spiritual body. If there be a natural body, there is also a spiritual body, as it is written: 45 The first man Adam was made into a living soul; the last Adam into a quickening spirit. 46 Yet that was not first which is spiritual, but that which is natural; afterwards that which is spiritual. 47 The first man was of the earth, earthly: the second man, from heaven, heavenly. 48 Such as is the earthly, such also are the earthly: and such as is the heavenly, such also are they that are heavenly. 49 Therefore as we have borne the image of the earthly, let us bear also the image of the heavenly(1 Corinthians 15)
But put ye on the Lord Jesus Christ, and make not provision for the flesh in its concupiscences (Romans 13:14)
The sharp dichotomy of Rm 6:18-20 no longer obtains: we learn that sin and righteousness are a struggle where sin pays wages -- as we sin. We end up a torn man. Sanctification is a process, -- we put on Christ. St. Paul urges us along it. With fear and trembling we work it out, because it is God working with us. He works and we work. Freedom is the condition of a Christian man.
The truth hinges on a preposition.