Romans 6:19-20 — I speak after the manner of men because of the infirmity of your flesh: for as ye have yielded your members servants to uncleanness and to iniquity unto iniquity; even so now yield your members servants to righteousness unto holiness.
For when ye were the servants of sin, ye were free from righteousness.
manner of men — Paul uses the slave analogy so the Romans will understand. Slavery was common throughout the Roman empire.
infirmity — due to the Romans' iniquity, there's a good chance they won't understand Paul's point about the need to switch masters. Christianity isn't slavery, but Paul uses the term so they would understand.
flesh = weakness of human nature
iniquity = lawlessness — disregard and breach of the Law of God. 1 John 3:4 — Whosoever committeth sin transgresseth also the law: for sin is the transgression of the law.
Uncleanness (impurity) defiles the person, iniquity violates God's law.
unto iniquity — the effect of lawlessness
holiness = sanctification
Sanctification (hagiasmos) signifies (1) separation to God, as in 1 Corinthians 1:30; 2 Thessalonians 2:13; 1 Peter 1:2; (2) the course of life which benefits those who have been separated to God. This is its meaning here and in verse 22. Sanctification is a state which God has predetermined for believers, and is the state into which in grace He calls them and in which they begin their course as believers. On this account they are called saints, hagioi, "sanctified ones." Whereas formerly their behavior bore witness to their standing in the world in separation from God, now their behavior should bear witness to their standing before God in separation from the world.
As there are no degrees of justification, so there are no degrees of sanctification; a thing is set apart for God, or it is not, there is no middle course; a person in either in Christ Jesus, justified and sanctified, or he is out of Christ, in his sins and alieneated from God. But while there are no degrees of sanctification, it is evident there can and should be progress therein; hence the believer is urged to "follow after … sanctification" … W.E. Vine, The Epistle to the Romans, pg. 95-96.
free from righteousness — no relation of any sort to it
Sanctification is not sinlessness; it is to be set apart as sacred to God. Thus slavery to God and righteousness brings us close to Him in a blessed relationship — and this is what delivers us from the tyranny of the flesh. (Stam)