This has been dealt with before. As per usual, that great scholastic tool of the "distinction" to the rescue, in the service of truth ... along with some definitions.
Yes, Traditional Catholics (including Archbishop Lefebvre) have all concluded that the Conciliar religion is not the Catholic religion.
Definition (of a vague term): What do you mean by "belong to"?
Let's say I'm a Catholic and I see a Ukrainian Orthodox church down the street, but I'm not well educated on the subject and have heard of Ukrainian Catholic Churches and was told that it's perfectly legitimate for Catholics to go there. So I start going to that Ukrainian Orthodox Church under the mistaken impression that it's Catholic. I go there for years. Do I therefore belong to the Ukrainian Orthodox Church and cease to be Catholic?
Answer: No, and that's because my error is referred to as material error, an error or fact, rooted in ignorance. When I was a child, given NO catechesis, I had a mistaken notion of what the Immaculate Conception meant, and therefore held a heretical view on the matter. But as soon as I became acquainted with the proper Catholic understanding of it, my reaction was to immediately accept it and regret my former error. If I see $100 on a table and take it thinking it's mine, but it turns out that it belongs to someone else, while there's an objective injustice that I must rectify as soon as I become aware of the error, I am not formally guilty of the sin, since it's a material error.
Similarly in the Novus Ordo, there are some who continue to profess the Catholic faith and simply believe and have come to the conclusion that it's the Catholic Church; many simple folk who are not acquainted with the theological problems involved and think that the aberrations in the Mass are mostly due to their local priest's Modernistic tendencies, etc. etc.
Those who attend the Novus Ordo do not necessarily adhere formally to the Conciliar sect, even if they belong to it materially.
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