It follows that if the celebration of a Tridentine Mass is surrounded by circumstances that threaten to undermine my faith, then depending on the gravity of the threat, I may not attend such a Mass. That is why Masses celebrated by schismatic Orthodox priests may be valid, but the Church in her right mind used to forbid Catholics to attend on pain of grave sin, because, belief and worship going together, the non-Catholic worship threatened the Catholics’ faith. Now Orthodoxy has in the course of centuries caused huge harm to the Catholic Church, but can anything compare with the devastation wrought upon that Church within mere tens of years by Conciliarism? If then Catholics were forbidden to attend Mass in Orthodox circumstances, would not the same Church in her right mind forbid to attend a Tridentine Mass celebrated in Conciliar circumstances?