Perhaps in rash judgement, you missed the "s" in "heretics"?
I was never referring to mixed marrriages.
Second, your personal view, quoted at the top: "It's a sin for a Catholic to worship alongside heretics", was a very general statement which did not hint at any exceptions. Now I see that you would countenance an exception: mixed marriages. Any others?
Thank you for that clarification.
What about a Catholic praying and worshiping God at the bedside of a dying veteran at the VA hospital, for instance? Or a Protestant praying at the bedside of a dying Catholic vet?
I ask this because I am a volunteer to sit with the dying at the VA Hospice here, and I see it pretty frequently. None of this is the same as an "ecum4nical Mass," whatever that is. I would think that would be troubling to the theological principles of everyone all 'round.