So, Aya is a messenger of Jesus...
...Lord Tenma's secret identity has been revealed.

But, good-hearted jokes aside, I'm fairly certain there's a picture this is based off of; John being visited by one of the angels. Problem is... I don't have even the slightest idea of what it's called or who drew it.

I can't find a specific painting, but the themes point to it being a parody of various Annunciation paintings, where an angel visits Mary to inform her of the whole shebang.

Though here it kinda looks like they're just poking at Reimu for getting a bit chubby around the waist.