Really I’m fine with that. I’m just not going to use His Holiness.
Why? What on earth is wrong with “Sir”? - “Yes sir, that’s a lovely hat”, “No sir, I just ate”
Under any circumstances I can imagine, I’d say, “Hello.”
I can’t even think of the last time I addressed someone by a title.
If somehow I couldn’t avoid it, I’d say, “Sir.” Frankly, the chances of me remembering some point of etiquette like “Your Holiness” are so slim they make Calista Flockhart look chunky.
Your Fucking Fascist Peddo Protecting Scumbagness – if I was in a GOOD mood.
Thing is, that’s not an argument for someone like me to address the Pope respectfully, much less by a specific title. I don’t, even slightly, respect him, his beliefs, or his organization, so that part of your reasoning doesn’t apply. Not being a coworker, employee, family member, or otherwise forced to associate with him ( and given a choice I’d avoid him ) I don’t really care how much grit I toss into society’s machinery in this case. And as an American, atheist and liberal, I don’t believe for a moment he respects me; why should I respect him ?
An old-time newspaper humor columnist once was waiting to cross a street when a limo stopped at the traffic light right in front of him, and famed banker J.P. Morgan was glowering in the backseat. Our hero gave him a wave and said, “Hiya, toots!”.
Might be a bit disrespectful in the current context, but “Hi, Pope” works for me.
Now it’s a good thing I’ll never meet the Pope. The urge to say, “Hiya, toots!” might be overwhelming.
Using a big sticky label, with waterproof black felt tip:
Whaddayamean no respect? I specified First Class Mail, didn’t I?
If someone is such a pariah, so vile a person, that you must suspend the respect you have for the people who revere him (at least with regard to their affection for this monster), then etiquette, I believe, would say that all bets are off. Not to Godwinize the thread, but if Hitler demanded to be called “Your Virtuous Paragon of All That Is Good,” one could rightly dispense with such a title if one was inflicted with his presence. Too bad if the German people circa 1940 would be offended.
If the Pope is such a person for you, then I don’t believe courtesy demands that you ignore a deeply held conviction. That said, most folks, I’d wager–even atheists–don’t care so much.
Well, for Der Trihs, he has NO respect for the people who’d revere the Pope (or anything non-material) in the first place, so it’s no problem for him. 
Sir would be the most respect I could muster, I think, if I was pressed into meeting him. Frankly I don’t respect him, or his beliefs, and I would avoid him if at all possible. Atheist here.