Romans 3:19-20

Romans 3:19-20. Now we know that what things soever the law saith, it saith to them who are under the law: that every mouth may be stopped, and all the world may become guilty before God. Therefore by the deeds of the law there shall no flesh be justified in his sight: for by the law is the knowledge of sin.

The law can convict and condemn, but it can never justify the guilty. Its special work is to prove that they are not justified in sinning, and to stop their mouths from uttering any excuse for their sin.