Sorry but I have to disagree with your example. Too often people don't know what is in their holy books or doctrine and it's not disrespectful if it's done properly such as "What about Sally 2:23 where it says that..."?
I've never had people take offense when I cared enough to actually look into someone else's belief system enough ask a question from that person's frame-of-reference.
If it is properly addressed, yes. That was the first dialogue. If it's:
Christian: I believe jesus saved us from our sins
Person X: I do not believe this is so
Christian: I can prove it from MY scripture
Person X: I can rebut it using the same foundation/source you gave me
And you can debate back and forth about who interprets scripture best.
If person X says instead: Jesus did not save us from our sins because it says in MY book that he does not, here is the historical evidence, and continue to disprove your logic by offering what I believe, it overrides a honest difference of opinion.
It literally says "the bible is false" to a christian with whom, depending on the conversation, would consider that a huge insult especially when it's proven by an outside source that has no connections to Christianity whatsoever.
It's like a spit in the face.
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That is very different if Person X decided to prove her point by christian scriptures or christian tried to prove his point by using Person X's scripture.
But if you are using your back up source to prove your point and
fact of another person's faith you haven't practice nor know intimately enough to common beyond opinion, it's best to be safe and say "this is my opinion.." or "I disagree because of this..."
but never saying to the christian (as a buddhist, for example), "no, your scripture doesnt say that. It says this. I know this because my suttas say this and you have misinterpreted your scriptures and not everyone believes in their own scriptures correctly, so that is why I am telling you the truth."
That this is not a healthy conversation.
But I don't know what can be done. Christian atheist debates do this all the time (atheist using christian scripture to prove christianity is wrong about something inherit in christian belief he disagrees with)
Arguments are based on emotions "You are wrong.'
Debates are based on evidence/what one can prove; "I am right."
Discussion way both and want to learn from both person's point of view. If christian, atheist, and person X does not want to do this, there is no use in debating.