To say that's where the conversation needs to start assumes they need to argue it out and one of them needs to change their mind.
But why? If they're both comfortable in their belief systems, why can't they leave it at that?
Just supposing for a moment that I accept the premise that comfort is the deciding factor here...
Consider what Christianity says: that if you don't fall for its tale of bloody sacrifices, then you not only will but SHOULD be tortured forever. Consider what "forever" means compared to, for example, a thousand years... or a trillion years... or instead of that many years, that many times the entire history of this universe not just past but also future... or instead of a trillion, make it 999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,
999,999,999,999,999... squared... or cubed... or to the power of
itself. It's still a finite number, so it's essentially
nothing compared to infinity, which is the amount of time that any non-Christian is supposed to spend in agony more horrific than anything that could be imagined from our Earthly experience, worse than any human has ever had to live through for even an instant in this life.
And that's not even just an unfortunate fate that they would wish for us to avoid. It's what
should be done to us, because we
deserve it. That's the Christian idea of
justice. That's utterly psychotic.
Anyone who is "comfortable" with a God who works like that (nevermind proclaiming
love for such an immeasurably, unimaginably, incomprehensibly vicious monster) is utterly psychotic. So, anyone who actually believes in such a God must be either psychotic or not comfortable with it. Since most of us never interact much with utterly psychotic individuals, it's usually a pretty safe bet that, unlike what your question supposes as a premise, the people we meet who call themselves "Christians" are in fact not utterly psychotic, and thus NOT at all comfortable with Christianity's God and that God's behavior and attitude. For those who actually buy it, getting them to see that it's all fiction would be freeing them from living in a constant nightmare.
For those who don't buy it but inaccurately call themselves "Christians" anyway, the self-misidentification is and indication that they're not comfortable with admitting what they really think, so again the premise of your question doesn't apply and there's no reason not to try to get them to see that it's OK to admit what they really think.
I have no problem treating people with respect, as long as they treat me likewise.
Thinking that someone deserves eternal torture is not respectful.