Defining our terms, it's easy to see that "continue in sin" = still live in it.
If Paul’s doctrine had been that salvation depends in any degree upon our good works, no such objection to it could have been made. We know from Romans 3:8 that it was brought against the apostles. We can also gather from Galatians 5:13, 1 Peter 2:16 and Jude 4 that some gave occasion to the charge. But it is a total perversion of the doctrine of Grace. That is what the apostle here proceeds to show. Chapter 1 establishes the sinfulness of the Gentiles. Chapter 2 describes the sinfulness of the Jews. Chapter 3 sums up this line of reasoning, and shows that only by free grace and justification by faith does the δικαιοσύνη θεοῦ come. Chapter 4 portrays Abraham as proof of this doctrine. Chapter 5 demonstrates the blessings that flow from it. Then Chapter 6 takes on this error.
Continuing to live in sin once one has died to it is no more logical than to try to live in a house after you’ve sold it. A death row inmate set free, but who refused to leave his cell, would still be legally free even if he didn’t walk in it experientially. His not leaving the cell would not argue against his freedom, but against his understanding of Freedom. This is the situation of one who is saved but continues in sin. His freedom is not lost or retracted, but for practical purposes, it might as well have not been granted.
This would, in fact, never happen in the real world. Its opposite, however, is the actual state of every unregenerate man. Adam alone would have retained memory of his former state of Original Righteousness; the rest of us have been born in prison. Our natural enmity toward God causes us to deceive ourselves into imagining we are free when, in fact, we are in bondage. It is a billion times easier psychologically to convince yourself that you are free when you are not (sort of a moral “Stockholm Syndrome”), than to convince yourself that you are bound when you are actually free. There is nothing to motivate that delusion.
So to get caught up on the question of whether salvation can be lost or not (although one side is definitely correct), is to miss the whole point of this passage. The hypothetical “Christian” who accepts the Lord as Savior and then proceeds to continue in his same life of sin based on the assurance that once he is saved, he can never sin away his salvation, is no Christian at all. We would deny the very existence of such a person. Scripture categorically denies that those who willfully persist in sin were ever elected in the first place because we are elected through sanctification unto obedience. This is “putting God to the test,” and no Christian would ever do this! What kind of “Christian” makes this hypothetical experiment to see whether or not he really can remove himself from Christ’s hand? The notion is simply asinine. This is like a pardoned death-row inmate trying to see whether he can still get executed!
This false conclusion overlooks the following:
1. the fact that a Christian has a renewed nature;
2. that justification is more than just forgiveness of sins;
3. the difference between a lapse in Christian character and living in sin; and
4. the fact that justification by its very nature leads on to sanctification.
1. A Christian has a renewed nature.
2. Justification is more than mere forgiveness of sins. This is the mistake Whitefield pointed out in Wesley’s theology. Justification is a change of status before God: from “in sin” to “in Christ.” We are translated from the power of darkness to the kingdom of Christ. Hence it is a change wrought in us by God whereby we are changed from in sin to in Him. It may even be considered a change of citizenship (Colossians 1:13).
Furthermore, justification, when God imputes Christ’s righteous to us, has a built-in principle of righteousness (Jeremiah 31:31-33).
3. There is a difference between a lapse in Christian character and living in sin.
It dishonors God when we cower in fear when we sin rather than coming boldly before the throne of grace. Christ commanded us to forgive 70x7 a day! Can we expect less of Him than He commands of us? It should be obvious that I am not advocating sin with impunity. That is precisely what Paul's foolish opponents accused Him of teaching. There is a difference between a cowering, servile fear of God that runs and hides from His presence and a healthy, sanctifying fear of God that is confident in the merits of Christ's atoning sacrifice.
4. Justification by its very nature leads to Sanctification.
The whole point of the New Covenant is to ensure holiness (Jer. 31:31-33). Where the Law lacked the power to work holiness in us, the sanctifying work of the Holy Spirit brings us by degrees into a progessively closer walk with God. Morever, God only justifies those whom He intends to sanctify (Rom. 8:29, 30). Any Reformed writer worthy of the name has pointed this out.
The order is “justified by faith,” not “justified by sanctification.” The later error gives birth to the objection Paul refutes in this passage! Sanctification is built on the foundation of justification: not the other way around.