Pulling off a believable 'Unlikely Couple'

So Character A happens to be Deadpan Snarker who happens to be a bit of an anti-theist due to some major qualms he had with the religion he was raised in. Initally, he's kind of a dick about it, but he can tolerate friends that happen to be somewhat religious.

Character B happens to be the equivalent of a missionary (but with a greater emphasis on charity work) in their world and is very devoted to their religion.

They're forced to work together and initially really don't like each other. They slowly manage to become friends and eventually go through a Relationship Upgrade. Character A learns how to be less of a JerkAss and be more accepting.

• Any comments/tips for pulling this off?

Any help would be appreciated

Since I don't know much about the actual story, it's perfectly realistic to have a more devout person end up with a less-devout one, even if the less-devout one happens to be a jerk.

Not every couple needs to share the same level of piety, and there are lots of real-life couples who don't even share the same belief system.

Just make sure not to make the religious one a Morality Pet for the less-devout one, because that could fall into Unfortunate Implications.

So just try to not make them too extreme (especially not making one of them too much of a Hollywood Atheist)?