Fundamentalist are basically fearful people. They want quick one-liner buzz-phrases and formulas that they can cling to as fear pacifiers. These are always fractional truths and become just like superstitious charms.
So, one must confront the fundamentalist with a more ostensibly potent jingoistic trinket than the one they are holding. They must be progressively forced to replace one with the other to eventually come to realize that their entire belief system of one-liner theology is just wrong.
What I am proposing is shock therapy. It is a last resort when reason fails. It should only be used with emotionally stable individuals and in extreme cases where the person is so blinded by hatred and bigotry of Catholics that this is the only way to get their attention. I hate personally to stoop to the fundamentalist level but sometimes this is the only way to engage certain individuals. Its like ‘fighting fire with fire’ and it invokes a benevolent behavioral change through stress adaptation. The objective is to awaken a small corridor of objectivity within a subjective mind. If we can do that then a person has a chance of being enlightened to truth by God’s workings through the Holy Spirit.
A fundamentalist principal fear is they will go to hell if they do not ‘truly, truly, truly, truly, truly, really, not-kidding… believe that Jesus will save them.’ Their 2nd biggest fear is that we are in the end-times and the anti-christ is possibly lurking under every rock, government leader or neighborhood Catholic Church in the world. Their 3rd biggest fear is they are going to miss the rapture buss (which they equate with not being saved) and will have to live through all the anti-christ’s tribulations. Their 4th biggest fear is that The Catholics (and what their own inner-conscience tells them) just might be right; there might really be no free-lunch to sin and OSAS just might really be a doctrine of man. Fear number four scares them since it means they might really need to repent for each sin and make restitution or face the consequence of hell; and in the bliss of OSAS they totally lost count of all their sins.
So when reason fail, fight fire with fire; fear against fear.
Just tell these people to take a bigger bite of The Word and read all of John 6:41-6:66. Then tell them that Jesus marks those that rejected His teaching about Eucharist with the sign of the beast just like he did in the verse number used in John 6:66 that condemns them!
You may have to go to confession after this one though. To avoid the possibility of that reframe it as a rhetorical question : “Did John mark those people who rejected Jesus’ word just like He did the number of the beast in John 6:66”?
This will immediately force them to simultaneously confront their top four fears and through panic create a real opportunity for them to reevaluate their entire fundamentalist philosophy (which is a false religion of man).
The only danger here is if one of them really is anti-christ - so better have your blessed religious medal on when you do it.