I'm sorry, with all due respect, but if their believes weren't heretical they wouldn't have left the Church. How can I condone the teaching of error in the name of ecumenism?
It is by speaking kindly and avoiding kneejerk buzzwords that you do it.
And remembering you will not win people’s hearts and minds if you hit them over the head with a 2 by 4. Tone matters extremely in online conversation. You might reinforce your belief that you are right, but you stop people from listening to what you have to say, and therefore you communicate not truth, but attack, even if there is truth in the statements. Because they will not hear it.
By realizing that "error" is a spiritual concept, but a judgment passed by a collective egoic mind.
With all due respect:
1) There's a huge difference between disagreeing with someone's beliefs and misrepresenting those beliefs. The latter is a form of false witness against your neighbour. It's potentially a matter of mortal sin. (No, I'm not accusing you of doing it.)
2) There's a huge difference between refusing to condone the teaching of error and ridiculing a person for passing on what he was taught (and what his parents and grandparents were taught before him), even if we believe that he was taught in error. That, too, is potentially a matter of mortal sin. (Again, I'm not accusing you of doing it.)
I think we've all seen these offenses committed on this forum; I think we all need to examine our consciences.
No one’s asking anyone to condone error.
But haughty self-righteous, critical-spirit judgmentalism; bad-faith sanctimonious hostility and abusiveness are not fruits of The Spirit.
And, timing and context are important as well as tone.
I think there IS a time for passers by to arrest the child-molester or murderer or wife beater etc.
But for a grubby homeless alcoholic to rise out of the gutter and self-rightously with great hostility, smugness, pique, sanctimony etc. insist on straightening the collar of the nun in the white habit . . . is . . . a bit much.
. . . more than a bit.