Fidei I would dispute that definition. Mormons might believe that, even JW’s might i’m not sure, but they’re not Christians. There has to be at least a BASIC belief that Jesus IS in fact both God and man, not just a man or not just God or whatever else. Or not just one god among many gods. I still have trouble calling some types of Protestants even, Christian, especially those who aren’t baptized etc. But I think Protestants who do believe in a correct Jesus, the true Jesus, as much as they can without the Church, are Christians in a certain sense. Not in a full sense though. I would say that about certain types of liberal ‘Catholics’ even. At times in my life I would have said that about myself certainly. Again, I’m saying this generally, not about individuals. But to get to the original question to your Traditional Catholic friends, you need to tell them first of all that the most Traditional thing a Catholic can do is obey the Church. And the Church has said this in Vatican II about 'seperated bretheren. Maybe they could have doubts, or struggles about it, but that’s why we need a Church to tell us these things, and this is why obediance is one of the key virtues, because we don’t know everything, we can’t. And we cannot rely on ourselves to make up doctrines or beliefs or whatever else. That’s why we had to have revelation from God about Himself, because just about EVERYONE on Earth had it wrong. That’s what I would say anyways.