Archbishop Lefebvre Advised his followers not to be in Communion with Catholics subject to the Roman Pontiff, Evidence of Schism per Can 751
"It is clear that with all those who leave us or who have left us for Sedevacantism or with those who want to be subject to the current hierarchy of the Church while hoping to keep the Tradition, we can no longer have relations with them. It is not possible. We say that one cannot be subject to ecclesiastical authority and keep Tradition. They claim the opposite. This is to deceive the faithful. (…) we want to be absolutely free from compromise both with regard to Sedevacantists and with regard to those who want to be subject to ecclesiastical authority."
-Archbishop Marcel Lefebvre
Conference in Flavigny, December 1988, Fideliter n° 68, p. 16.
Konstantin: So basically the Church no longer exists because being in union with the ecclesisastical hierarchy and holding on to Tradition is no longer possible. So many of these quotes not only reek of schism but of heresy.
...
Question:
17) Can we go to the Masses of the “ralliés” [the ED communities]?
No, you can't go to ralliés Masses
firstly because attendance at Mass is a public profession of faith and this profession of faith is altered by the ralliés,
secondly because attendance at ralliés Masses leads to a relativization of doctrinal differences,
thirdly, because such assistance leads to contacts that endanger the Faith.
[Konstantin: Here Lefebvre's quote starts]
"They also say: the Mass is fine, we are going. Yes, there is Mass. It is fine, but there is also the sermon; there is the atmosphere, the conversations, the contacts before and after, which make you slowly change your mind. So it's a danger and that's why in general I think it all goes together. We don't just go to Mass, we frequent an environment.
There are obviously people who are attracted by the beautiful ceremonies which also go to Fontgombault, where the old Mass has been resumed. They find themselves in a climate of ambiguity which, in my view, is dangerous. As soon as one finds oneself in this atmosphere, subject to the Vatican, ultimately subject to the Council, one ends up becoming an ecumenist."
Exclusive interview with Archbishop Marcel Lefebvre – Fideliter n° 79 of January-February 1991
https://laportelatine.org/formation/crise-eglise/ecclesiadeisme/catechisme-des-verites-opportunes-les-rallies-vus-par-mgr-lefebvre-abbe-f-m-chautard-8-juillet-2018
Translations provided by Google Translate and modified by -Konstantin
Addendum:
Konstantin: For context I would like to point out that this wasn't a point of view that was only held in the years after the consecration. It is from a "catechism" posted in 2018 on the SSPX's official French District website. The author is Abbé François-Marie Chautard, (former?) rector of the FSSPX University Institute in Paris.