When i played as christian daimyo i chose Mori, but any clan would probably work. Otomo the best. However if you abandon buddhism as Uesugi you will lose their bonus as better monk units etc. I recommend not to do that. Also if you want to become christian right away choose Shimazu or Otomo, (Otomo being a dlc) the reason for shimazu is that they don't need the event to build nanban port. I recommend any south western clan for christian such as: Otomo, Shimazu, Chosokabe and Mori.

]Playing as christian Otomo is annoying because everyone from the start will either hate you or not like you. And Otomo starts at war with 2 clans:Ouchi and Shoni.
So a must do thing is to build a medium strong navy(depends) to block the strait where ouchi can invade you. In the start you get a gun boat, a powerful boat so don't underestimate it. Use this boat to blockade the strait and focus on shoni. When i played as Otomo, Ouchi made peace with me the second they died, so it's hard to make peace with them. When you have Shoni's provinces try to blockade even longer because you are gonna need Kyushu.(the island you start on) Since converting takes a while, so in the mean time block ouchi's ports so they can't build ships to destroy that strait garrison. When you get the money make a nanban quarter for gun boats.(these wrecks havok on boats because they got cannons.) Put 1 or 2 cannon boats on the strait, really good for blocking.
After you have got Kyushu and it does not need more garrisoning you have to options: Invade honshu (risky) or invade Shikoku.(more secure) The reason Shikoku is more secure is because after converted and all that stuff you have to main objectives: Kyushu and Shikoku. Now you can invade honshu with 2 armies (very recommended) and destroy with your armies. After you get a foot hold on Honshu take the armies in a combined attack on Kyoto. (only need 1 if you are good) once Kyoto has been captured take the armies and secure foothold around Kyoto incase something goes downhill. From there on take the armies like this, the armies must be good also: Take the armies on a north and south path. Hope the Otomo Guide Helped!

Playing as Shimazu and converting can be quite quick as Shimazu don't need to wait for the event. Historically in 1543 European traders came to Tanegashima Island(south of kagoshima) and Shimazu probably found them.(i don't know,thats my theory) Shimazu starts with heavy Katana Samurai so use them in gunpowder armies because gunpowder ashigaru are not strong in melee, its about 3rd rank they get melee as yari ashigaru. But after you have the Nanban Port you should assault Kyushu and try to ally Shoni or Otomo (shoni usually kills otomo and becomes christian) After this convert Kyushu and do the steps i told you to invade and should go good from there.

The differenace to Otomo and Shimazu is that mori will have to wait so capture the province north of your capital and upgrade gold mines for better income because you are gonna need it later, after you have killed Amako clan or made vassal. Upgrade the provinces and get a medium skilled army(samurai army would cost to much, believe me) to protect because kikkawa or whatever they are called will make war if you don't so protect your borders. When you can become christian don't go right away unless your daimyo has 4 or more honour because 2 or less honour will make unhappyness in all provinces. After you have become Christian convert the neighbors and and your own provinces because it dosen't matter later if they hate you, since realm divide will make everyone declare war against you. When you best bud' declares war on you, Ouchi shall die like the buddist scums in japan! After this go for Kyoto (try to ally with a clan in Kyushu) and Shikoku. the reason for not going Kyushu is because it does not really interest player when playing as Mori.

The Chosokabe is strong in archery, firelocks enemy. When playing as Chosokabe take shikoku before invading anything because you will have privacy, for awhile. The Chosokabe must wait for the event before going christian. If you want to go before the event go for Bungo province, Otomo capital with nanban port and you will have to destroy that one before to build in Tosa province. When christian try to make a stable economy and army because you are gonna need it. When these are achieved move to Kyushu and capture it, then convert it and move to honshu. After Honshu is Kyoto on the capture list,take it with an combined force or one army and you are shogun, Chosokabe Shogunate! As bows have a much bigger advantage than guns being range and terrain. Bowmen have a range of 150 while guns have 100 execpt Tokikahs Tanegashima which has 125. The second being Terrain because bowmen can shoot over hills and fort walls while guns can only shoot in line of sight of target. But the Advantage guns have to bow is Accuracy.

Being a Christian clan is tough, especially if you are not used to convert alot of provinces. The advantage to Christian clan is that they have:gunpowder units and gun boats. The Otomo has Portugese Tercios but thats really their special thing. Christian disadvantage is that you may not recruit: Monk, Monk Unit such as Naginata Warrior Monk and etc. It will be Longer to Convert Japan as the peasents will take a while to convert from Buddhism to Christianity.

EDIT: You kill everyone who isn't christian, and if you can, try to find a strong ally like Oda or some other clan.
EDIT: Missionarys are perfect for creating revolts, this also levels them up.

So End of Guide and tell me if it was bad or good. If it helped you or not and maybe some other stuff. This was my first guide so tell me what was bad or good and feedback would be nice.