Disclaimer: I don't think I even own the idea, I thought of this when I was reading Alowl's GO poetry.
Summary; I wasn't feeling Christmassy so I decided it was time for this. No slash. Science of the Discworld reference in footnotes, kinda…
It was unusual when Aziraphale wasn't at the bookshop in the evenings, reflected Crowley, unusual but not unheard off. Even so, he got out of the Bentley and strolled down the street. Of course the angel could be anywhere but Crowley had all night if he wished. Down Below were usually content at this time of year anyway with greed, envy, gluttony and occasionally wrath everywhere, souls were practically delivered to them with a red bow.
Then again, there were those who redeemed themselves, but, Crowley reasoned, you can't have everything and Hell could be convinced it wasn't his fault.1
Crowley was quite the actor.
It hadn't rained, so the streets were dry, odd for December and there were no puddles to reflect the neon Santa and snowflake signs, no pools of rainwater for a forgotten lover to watch the ripples of his own tears as he felt his heart smash within him2. Snow would stick, if it fell. Though the weatherman had sorrowfully announced that there was a one in a million chance of a white Christmas.
"Can you light this candle?" Crowley halted in his tracks, a little girl held up the thin Christmas candle on a cardboard disc to stop the wax getting on her hands. Her mother ran over and apologized profusely. She led her daughter away, telling the child that her candle was supposed to be lit during Mass, and not to bother any more strangers.3.
Crowley would've mentally kicked himself, Christmas Eve Vigil Mass, how could he have forgotten? Avoiding the holy water on both sides of the door he peeked inside. There was the angel sitting near the front, lost in the thoughts of the otherworldly.
It struck a chord with Crowley, though he didn't understand it at first. He thought of all the deeds he'd done, and was struck suddenly with this feeling of guilt. A human emotion, he noted. But then again, he'd chosen that hadn't he? He lived among humans for millennia, of course he'd become like them. All the nudges, they never were more than nudges really, on to the road to perdition. He slid into the back pew and prayed. As though he'd never prayed before.4
He felt his heart lighten of the burden he hadn't even known was there. He left the pew and dipped his hand fully into the holy water font at the side of the door. he didn't even flinch. With his other hand he removed his sunglasses the shiny incense burner an altar boy was carrying told him all he needed to know. His eyes were hazel; he was once again an angel.
As the last bars of the finishing hymn died down, he walked to the front of the church where people were lingering near the manger. He stood beside Aziraphale, who smiled and faced him.
"Hello angel."
And Hell swore.
1Crowley was in fact the inventor of the lie. Unbeknownst to himself, Aziraphale had earned the title of Inventor of the Not Quite Truth (also known as the Tell-them-as-much-as-they-need-to-know) back in Eden
2The lover in question eventually gave up on the whole poetic scene, called his mates and ended up feeling much more cheerful. There is a time and a place for emotional displays, but not near Crowley.
3She later became a professional athlete, go figure.
4Of course he had prayed before, praises to God and all that, but it was AGES ago.