Ask his lady friend out.

Well, alrighty then. Too bad no one taped the episode, having him watch his behavior might help.
As I was reading your tale I was wondering how the "new" girlfriend was taking it all in. She responded well, though for myself I would have not made any offer of returning in exchange for him behaving better.
Tell him all he needs to do is start believing in himself. The woman JUST explained how he can be a better person and pretty much covered everything in C.

It might pay to sow more seeds of doubt about his own inerrancy. Said female friend has already started the process.

"Did you wonder how I managed to quit smoking without God/Jesus?"

Is this what "General and Hellos" has become?

Lesson one. Nobody is responsible for anyone else.

Lesson two. If he only wants to quit smoking out of embarrassment, and to look better in the eyes of other people (or his lady friend), he is unlikely to succeed. A person can only change if they really want to change for himself.

I am still laughing my ass off that hi9s lady friend turned on him.

You are the one who said you are not one to duck out of a challenge, and boy did you step in the flaming bag of dog poo with this one. But if you are feeling childish, you can give him a set of funny rituals to try. Like staring in the mirror and repeating the oath to the Royal Order of Siam to help strengthen resolve. See if he catches on.

Step one stop believing in a sky fairy.
Tell him to get back to you once he has mastered that one.

Hmmm... I'm going to step out for a smoke.

Sounds like you two are on your way to being best mates! Pound enough beers and keep the gal handy, you may have just have stole one from Jesus!

Give it ten minutes.....he will revert to his true nature and you will have been spared what was sure to be a very unpleasant ordeal.

In my world it is the Will and not will power.

Move house.

Just remember, that you can lead a theist to reason, but you can't make him/her think. Good luck! Maybe you can teach him that life without god can succeed.

I wouldn't associate with anyone that unpleasant in any way.

There was brilliant conversation in there somewhere?

I got nothin'.
Good luck with ALL of THAT.

I can’t even.

The ongoing story of Evangeloon.

Definitely a challenge at best, however, as the old saying goes, a leopard can't change its spots.
You can tell him, but you can't change him, he has to do that, he needs to be constantly aware of everything he does and say, it's on him, you can't teach him shit, only guide him, and that depends on if he follows your lead or become a total ass about the changes demanded on him.
Personally, I would just leave him be.

Wow, just wow.

Lol, I've been there. I failed horribly though. I made the assumption that they wanted to learn from something I believed. Really they were looking for the first chance to find fault and be done. I figure if you hold strong and keep it simple with a huge dose of scepticism you might do better than I did. But I've learned not to look to me for wisdom.