Those of us in the mission field who witness to Catholics in the truth of Gospel should realize what a Catholic really fears. And that is that you might be actually telling the truth.
If your witness holds to Scripture as the authority for the Gospel, the Catholic has no place to go but to grasping at straws in this fear. “You follow Luther!” is the sentiment in this fear. Empirical proof can be brought out showing that Luther had NOTHING to do with your conversion into the Kingdom of Jesus Christ, and by this internal fear the Catholic will still sadly scream, “You are following Luther!”.
It is a natural fear. If you have invested yourself in anyway into a religion such as Catholicism, made sacrifices, etc., the last thing you want to realize is that you are being deceived. So a defense mechanism is immediately drummed up, irrational statements are made in order to satisfy yourself.
So the Catholic reaction is always a rejection, as in someone who is being scammed by a scammer saying “No, how dare you try to say I am this stupid, I am NOT being scammed”. Then the protestant strawmans are used to defend the Catholic faith without realizing they are exposing their faulty standard. The irrational and faulty standard is “my pope is better than your Luther”. When the standard is always an eternal Holy Almighty God.
The fear will always lead a Catholic to say, “no, you are wrong! I believe and follow Jesus Christ”. But then the version of Jesus of the RCC must be dissected to show them they are actually following a persona of Jesus which doesn’t exist. A Jesus who is not a sovereign God, a Savior, a Redeemer, but basically a traveling salesman. A Jesus who puts a pope in His place as a standard of holiness, not His own inherent holiness and righteousness as the standard for the sinner. One Catholic for example said that “Mary can change Jesus’ mind”. Another said that he “would pay his own punishment for sin”.
As with every world religion, fear is the driving force to keep the congregant in their chains. The problem is when you discern the sinner in these world religions, they don’t fear God but the world religion. It is definitely PRIDE which is shown as they allow that fear to shut down any thought, any discernment that one would bring forward. So the reaction is to attack Scripture, attack its sufficiency, attack every non-Catholic, use fallacies and rationalizations and innuendos as broadly as possible in order to quell this misplaced fear.
Roman Catholics really do fear the truth. Remember this when you witness to them.