The conscience that is uncleansed can be a very powerfully, deceitful thing. All the mind can do in that state is produce dead works and fruit unto death by churning up the most self-righteous, zealous, sincere deeds to recommend one's self to God.
When people preach law without the gospel it drives the sinner more insane to produce their own righteousness because their natural conscience is stirred and their burden of guilt grows to pressure them for an action to perform to reconcile themselves to God.
People have been deceived many times over into false professions because of this natural conscience conviction. This happens in the heart WITHOUT the work of the Holy Spirit. That is why it is so dangerous and is a strong weapon in false religion's arsenal.
This is the chief means that is used in the Altar call and sinner's prayer system of calling people down the aisle after a false gospel message, coupled with an emotional appeal to get something done to "get saved". This is nothing short of Humanism and is part of Satan's lie. It is indeed a very, very strongly deceiving set up. Beware of what the guilty conscience can get the unbeliever to do if the gospel is not their to explode their wicked intent of the heart. Thank God for illumination grace the Spirit gives the sheep when the true gospel is delivered to them.