The Scripture cannot be broken (John 10:35). This means that there will not be contradictions in God's Word. Jesus says that not one jot or tittle would pass from the law (Matthew 5:18). Paul says here, "Do not sin," and sin is the transgression of God's law (I John 3:4). Nonetheless, Protestants say that the law is done away. This raises a contradiction.
If Jesus' death combined with the New Covenant does away with the law, then there is no such thing as sin, and Christ died in vain—especially as far as those who have lived since His death are concerned. Romans 6:1-2 states plainly that Christians are not to sin, that is, break God's laws. Therefore sin—and thus God's law, which tells us what sin is—must still exist.
It cannot be both ways. If they say that the law is done away, then in the biblical context, it is logical to conclude that there can be no sin. It is therefore illogical for them to claim that it is still wrong for a person to murder or to commit adultery because those sins would not exist without the laws that determine they are immoral or illegal acts.
But the true answer lies elsewhere: Their conclusion that the law is done away is wrong!