There are many, many passages in Scripture that describe the people for whom Jesus died. Included in those descriptions are many passages saying that he died for sinners. Limited atonement asserts that Jesus died for sinners, just as general ransom asserts that Jesus died for sinners. Therefore, the mere fact that Jesus died for sinners does not defeat either position.
With specific regard to Romans 5:6, that verse tells us about the qualities possessed by those for whom Jesus died. Limited atonement and general ransom both affirm that those for whom Jesus died possessed the quality of being "ungodly" or "sinful."
But Romans 5:6 does not tell us about the identities of those for whom he died, or about the number of those for whom he died. These are the issues on which limited atonement and general ransom disagree. Romans 5:6 support the general ransom theory only if it suggested that Jesus died for every person who ever possessed the quality of "ungodliness" or "sinfulness."
In actuality, the group that Paul had in mind is pretty clear. He began Romans 5 by identifying his subjects: "Having been justified by faith, we have peace with God" (Romans 5:1). He ended this chapter in the same way: "While we were yet sinners, Christ died for us. Much more then, having now been justified by his blood, we shall be saved from the wrath of God through Him" (Romans 5:8-9).
The "ungodly" whom Paul specifically had in mind in verse 6 were believers who had been "justified by his blood." He was speaking of himself and of his Roman audience, and by extension he was speaking of all believers. But he was not speaking of every person ever, since not every person ever is justified by the blood of Christ.
To put Romans 5:6 in context, Romans 5 is about the comfort and security that believers should have in their salvation. Paul's argument is that we did not merit salvation, and therefore that our future glorification is certain. That is, since Christ died for us while we were still his enemies, there is no way that he would withhold any blessing from us now that we have been reconciled to him. After all, friends always receive better than enemies.
Explicitly, then, this text does not tell us whether Jesus died for a limited or unlimited number of people. Implicitly, it suggests that Jesus died only for those who ultimately come to faith. I infer this from the structure of Paul's argument. I would outline this inference as follows:
- Jesus' death for sinners demonstrated incredible good will.
- This good will assures those same sinners of future salvation.
- Therefore, all for whom Jesus died are assured of future salvation.
This is the same kind of argument we find later in Romans 8:32-33: "He who did not spare his own son, but delivered him over for us all, how will he not also with him freely give us all things? Who will bring a charge against God's elect?" Paul's point seems pretty clear: If Jesus died for you, then you are elect and you will certainly be saved.