My response is always, “Tell me what you think the Church teaches about that.” When they tell me, as they describe things, it’s not surprising they’re having trouble accepting it. I wouldn’t be able to, either.
Taking just the matter of papal infallibility, it’s remarkable how many people confuse it with impeccability, thinking we’re claiming that the Pope can’t sin. Or they imagine that it applies to every single thing he says, and every random thought that crosses his mind. Frequently, people have a too-narrow idea of it, not understanding that it belongs to the whole Church, and flows from an adherence to the Magisterium.
Some people think it gives a kind of super-power to the Pope, when in reality it limits him simply to teaching the truth.
So if someone says they can’t accept infallibility, or the Marian dogmas, or any other aspect of the Faith, let’s make sure what they “can’t accept” is what the Church really teaches.
