For the hardcore, they must be challenged and debated.
I would say it depends on where they’re coming from. If they’re Catholic apostates, I might reflexively be less charitable. But then again they are owed the dignity of active
listening. We know that many Catholics have left the Church because of very serious harms done to them by members of the Church. Think of the pedophilia scandal for instance. So even they are entitled to at least being treated with respect, and if they were hurt by clergy or other members of the Church, compassion.
For those who simply were born into Protestant families, it is wise to consider what the Catechism says:
818 "However, one cannot charge with the sin of the separation those who at present are born into these communities [that resulted from such separation] and in them are brought up in the faith of Christ, and the Catholic Church accepts them with respect and affection as brothers … All who have been justified by faith in Baptism are incorporated into Christ; they therefore have a right to be called Christians, and with good reason are accepted as brothers in the Lord by the children of the Catholic Church."
(my bold)
So while these Protestants may
objectively believe in heretical concepts, subjectively the guilt they bear is mitigated by time and circumstances.
If they a shred of intellectual integrity, then they will discern the truth.
Easy to say. But we are talking matters of faith, not trying to solve an equation. Assenting to something based on faith comes as much from the heart as from the intellect. When I reverted after 22 years of atheism, it was not something rational. It was a switch “clicking” in my heart.
Moreover as I’m sure you’d agree discerning Truth on the Internet is an activity fraught with danger. There is so much trash information out there, including sites that purport to be Catholic, that it’s enough to make one’s head spin. In a way I’m sort of glad I reverted when I did (1997) when I was less likely to try to find answers on the Internet. I had in fact been actively proselytized by some evangelical protestants (one colleague in particular). When I started reading the Bible at a very low moment in my life, I simply saw too many contradictions in what they said, and I fell back into my “comfort” zone and reverted to the faith of my childhood. I might have done exactly the same if I had been brought up Protestant. My colleague certainly argued until he was blue in the face what a mistake I had made. I was poorly equipped at that time to defend the Church, but I receive ample evidence from God that I was on the right track, and that sufficed for me.
Bringing people to the Truth is not an easy task! And it cannot be done by argument for the most part, that in fact often has the opposite effect, and we must think of the guilt
we ourselves bear on our souls if we drive someone away from the Church by arguing with them. My preferred method is simply to show what it means to
be Catholic through my own life, and let that slowly, and imperfectly, do the work; I might point out that this is the monastic tradition as well; and as you can see from my profile that is a spirituality that speaks to me.
This is a very touchy subject with me. My parents were pioneers of sorts, my father was Anglican and married my Catholic mother in the proper manner, that is with dispensation and the promise that offspring of the marriage would be raised Catholic, which is what happened, to the point that if my mother wouldn’t take me to Mass as a child because she didn’t feel well, my father would, so I rarely missed Mass and he also saw to it that I never set foot in an Anglican church except once, when my step-sister was married in 1963 (my father was a young widower with a daughter when he married my mother). We Catholics could learn something from his level of dedication to the promise he made.
I am also married to an Anglican woman. We did go through some bitter times until we both learned to let go, and realize that we would never win each other over by argument. We now respect each other and our faith walks, we don’t argue and we focus on our mutual love of God and how He has literally saved us and our marriage in the here and now.
So I can tell you as a practical matter, faith is such a mysterious, personal journey that attempting to convince someone that they are material heretics, isn’t going to fly very far. It’s going to occur in a manner far more subtle than that:
11 He said, “Go out and stand on the mountain before the Lord, for the Lord is about to pass by.” Now there was a great wind, so strong that it was splitting mountains and breaking rocks in pieces before the Lord, but the Lord was not in the wind; and after the wind an earthquake, but the Lord was not in the earthquake; 12 and after the earthquake a fire, but the Lord was not in the fire; and after the fire a sound of sheer silence. 13 When Elijah heard it, he wrapped his face in his mantle and went out and stood at the entrance of the cave. Then there came a voice to him that said, “What are you doing here, Elijah?”
1 Kings 19