You have Japan twice on poll. But I would say that Iran is most plausible non-Christian nation which would convert to Christianity.
The conversion of Japanese during the 16th century is really overrated, they did form a significant minority in places like Kyushu and Osaka, but nowhere near close to archive supremacy, let alone majority, in Japan, by the death of Nobunaga the number of Christians in Japan numbered 200,000, impressive if the total population of the country wasn't around 15 million, so 1.3%.Iran's the closest, so I vote for them.
Japan actually saw a lot of conversions to Christianity in the 16th and 17th centuries until the Shogun started cracking down hard on the religion, so if you have a more religiously tolerant Tokugawa Shogunate the country might end up becoming majority Christian. Of course, while the Tokugawa is technically after AD 50, I don't think it's really in the spirit of the OP.
Persia isn't too difficult as Christians were making sufficient inroads to "warrant" their suppression by the Sassanids. So options:What it says on the tin. You have any POD after 50 AD allowed - which is most likely to become Christian?
You have Japan twice on poll. But I would say that Iran is most plausible non-Christian nation which would convert to Christianity.