Like back when he was developing TD, he didn't know anything about Taoism, so he researched it and blam, the concept for TD was born.

The same thing might be applied for Christianity, albeit he added numerous references to it in his works subtly before, like the Youmu route in TD (where Seiga mentioned Jesus), or Yumemi (who threw a fecking ton of crosses that lead to Ellen ask her whether she is Christian), but have yet to add an explicit reference to that religion, like what role does it have in the lives of those who resides in Gensokyo.

When I searched Wikitionary for Japanese Christian terms, I found out this thing: 東方正教会 (Eastern Orthodox Church). Man, if ZUN is aware of that word, he would have turn it into a Touhou game right away. But he might have to alter it to 東方聖教会 so as to referring to all schools of Christianity without explicitly referring to the Eastern Orthodox Church in the game's title and themes.

In that game, of course Reimu and Marisa will be playable, but the other playable characters would be Yumemi (perhaps that will be her first return from the PC-98 era) and Byakuren / Miko (no affiliations with Christianity, but have been compared to Jesus in fanon (with Miko's case, she is compared to Jesus by Seiga)). The game should be numbered with a number such as 19 (contains 9, the number of the choirs of angels in Christianity), 23 (don't ask, just look at the digit), 24, 25, 27, or any number that contains numbers that are associated with Christianity.

Don't ask me about even if Mima could return in that game, since Mima is not even associated with Christianity, she is unlikely to appear in such a game.

Anyway, just pray that the 19th Touhou game will be of a Christian theme, have the title of 東方聖教会, and feature Yumemi and Byakuren / Miko as additional player characters aside from Reimu and Marisa. But please don't expect Mima to return in that game, since like I said above and I'm not responsible for a request of bringing Mima back to Windows Touhou.

P.S. My Touhou fangame project, Sanctum Puella Mari, could be a catalyst to encourage ZUN to officially create a Christian-themed Touhou game.