I would say joint religious prayer at any function for any reason is clearly prohibited as per se indifferentism (regardless of the subjective intention).
Well that isn't what I was talking about at all.
I'm talking about Pro-life action (counseling, talking to women, "rescue" operations, trying to shut down abortuaries, political action), and various apostolates like helping abused women.
But was this thread about ecuмenism -- gatherings between Catholics and non-Catholics? I thought it was a bunch of TRADS getting together in a TRAD-cuмenical affair.
No offense, but Mortalium Animos was clearly talking about Catholic vs. Non-Catholic -- not Trad vs. Trad. There is no "mortalium animos" equivalent which condemns Trad-cuмenism or neutrality in Trad politics.
So I brought up the topic -- what about specific Catholic products like pre-V2 books, Gregorian chant, statues, mantillas, etc. -- do such companies need to "pick a group and stand on principle", sacrificing 5/6 of their potential market -- or otherwise be considered lukewarm? Or do these groups have "no dog in the fight" on things like the Pope question, the best Trad group to be a member of, etc.