Suppose you know a person who says he or she is trusting in Christ alone for justification. Then suppose you see that same person commit a sin. You have "found" them a sinner, meaning, they have been seen that they sin. They are not a former sinner at all. They still sin. Would discovering their sin mean that the Jesus of this justification Gospel message encourages people to sin? Definitely no!, Paul says.
Paul says that the opposite is true. It is the person who tries to re-build bondage to the Law who is the minister of sin! I destroyed my bondage to the law-system by trusting in Jesus Christ. So, if I try to go back to the law-system -- if I try to re-build the prison from which my faith unlocked my chains and swung open the cell door -- that's what makes me a transgressor! That's quite a shocking claim.
A justification gospel doesn't mean that I'm free to sin. It means that now I live to God (v. 19). I wasn't set free from the law so that I could sin. I was set free from the law so that I could not sin! As long as I was under the law, I could only sin. Why? Because the law condemned me, and as a result of my guilt I remained spiritually dead and without the Spirit of God. Until I receive the Spirit of God, I must sin.
So I need God's forgiveness, so that I can receive the Holy Spirit and, through the Holy Spirit, start living a righteous life free from sin. How can I receive God's forgiveness? By accepting, through faith, Christ's death for me. When I accept Christ's death for me, God credits my sins to Christ's cross. As far as God is concerned, it's as if I was crucified along with Christ. As if I was Christ on the cross.
Not only am I forgiven by faith. I also live every day in that same faith. The Son of God is the object of my faith. The justification gospel doesn't give you a blank check for sin. It replaces unbelief with faith, and faith produces good works.
If we get ourselves to heaven by good works of the heart or good works of the body, then Jesus' death on the cross was pointless. He didn't die to give us power to be good, so that we could earn our way into heaven. He gave Himself for us -- that is, He died in our place. Any message that says people can and do gain heaven by keeping the commandments is a message that rejects the grace of God. The justification Gospel -- and there is no other -- doesn't encourage sin. It is the only way we can stop sinning.