That is the point. Protestants by and large DO KNOW BETTER. They have chosen to deny the authority of the Catholic Church as the one true Church and have accepted instead whatever it is that they believe in its place
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It is their choice. I know a lot of protestants including some in my own family. ALL of them, the adults that is, and I mean all of them know full well of the differences between their churches and Catholicism and they fully embrace the differences as being correct in their favor. To them, we as Catholics are not Christian, probably not saved and are doomed.
To say that they are not responsible for what they believe is just plain wrong.
Pretend I am colorblind.
What if I told you the sky was green. You know that the sky is blue, it is a factual truth. But I only see green. You may tell me I am wrong. I may know from what everyone tells me that I might be wrong on the sky being green, as so many persons tell me its blue. But until I can see that the sky is blue, I will not believe it is anything but green. You can yell, you can scream, you can get as angry as you wish. But that sky would still be green to me.
You may ask me to just have faith and believe in the blueness of the sky, but, living in a world full of green, surrounded by green, I know nothing of blue, but that it is seemingly non-existant and foreign. How can you believe in something you know nothing of, or that is totally foreign to you. I would first have to experience blue, and then be educated and taught about it.
Until an individual discovers for him/herself that Jesus is truly present in the Eucharist, they are going to be in denial of it. They cannot yet see Jesus through the eyes of faith in Holy Communion. No matter if you believe it is the God given factual knowledge that Jesus is in the Eucharist, if you scream it from the rooftops, they cannot believe with only a partial knowledge and vision. Remember the blind man couldn’t see it was Jesus fully until his blindness was cured. He only had a partial vision at first.
We were blessed with eyes to see that the sky is blue. Maybe one day they will be too. But, until that happens, they cannot be blamed for seeing things the way they see them, when they do not know better. If it is green to them, they must live life as though the sky were in actuality green. That is how they will be judged, upon a green sky, not a blue.
Mother Teresa always said to help the Hindu become the best Hindu they can be. Meaning, we are called to help people live to their faith to the full, as their conscience tells them to. We must live according to our conscience. If someone has a faulty conscience, how can they be blamed for not knowing how to have formed it better? Sin is still sin, but if one does not know its sinful, it is no longer sin…at least not a mortal one.