In the aftermath of his surrender in August 1945, General of Army Douglas MacArthur acted as shogun. He had a virtual free hand from Washing to put in place any Constitutional reforms he saw fit. For example, making over a million tenant farmers into landowners, legalizing labor unions, institution a free press (with just one restriction: no criticism of MacArthur) and giving the vote to Japanese women. I must admit to feeling a certain malicious glee when I point out to certain feminists that MacArthur may be history’s greatest Suffragette. (Suffrager?)
MacArthur was a devout Episcopalian and saw Christianity as a positive influence on Japan. He encouraged missionaries (indeed, requested) to come to Japan to spread Christianity as a bulwark against Communism. The recently discovered diary of Defense Secretary James Forrestal relates that MacArthur told him he considered ordering Emperor Hirohito to convert to Christianity. Had he done so, many interesting questions would have been raised.
Until Hirohito renounced his divinity, he was the head of the Shinto church. This did not prove an insurmountable obstacle; the Vatican reached an understanding with the Shinto authorities allowing a sort of hybrid Shinto/Catholicism in Japan. (While I find that a bit bizarre, it’s much more sensible than some of the horrendous sectarian strife that has occurred in the past millennium.)
One of the first questions would be: If Hirohito converted to Christianity, which denomination would he convert to? Would the divine descendants of the Sun Goddess accept the Pope as head of his church, or, if he became Anglican, would he accept the monarch of England as the head of his church? Unfortunately, there was no option to convert to “white-label” Christianity, so that moment passed.