The Japanese during the wartime period referred to themselves as the "leading race" of the world. John Dower in War Without Mercy articulates that the Japanese due to the fact they turned to the West in order to industrialize following the destruction of the feudal regime in 1868 made Japanese supremacist ideas akin to those of White supremacism impossible. Instead, Japanese superiority was promoted as a moralistic superiority rather than a biological or intellectual superiority. This superiority according to the Japanese was buttressed by set phrases that championed virtues such as filial piety and loyalty to the Japanese imperial line. The emphasis on wartime purity undoubtedly would've made adoption of Western modes of life and religion unpalatable to most Japanese.
References to Chrisanity in media such as cartoons and the like would carry strong anti-Christian messages and images, as Dower describes:
Monkish renderings of Roosevelt and Churchill were defrocked and exposed as demons and warlocks. The Pieta was made a mocking (albeit sophisticated) parable of political futility, with Roosevelt as the Virgin Mary, Churchill the slain Christ, and Chiang a grieving disciple. The cross or crucifix was transmogrified. It became a bloody dagger, or a sign of death alone, just the marking on a grave. An unusually harsh cartoon in the government humor magazine several months after the war with the United States began combined form of the cross with the ideograph for America to offer a crucified Roosevelt as the new meaning of "United States."
Another point to discuss is the likely fact there would've been very few Christians in Japan up until the postwar period. Repression had long and deep roots, beginning in the Tokugawa Shogunate and forcing Japanese Christians into hiding as the Kakure Kirishitan, or "Hidden Christians" in the 1630s following the Shimabara Rebellion. Even with the reopening of the country in 1853, proselytizing remained forbidden until 1873.
Notes:
War Without Mercy: Race & Power in the Pacific War by John W. Dower
Imperial Japan at its Zenith by Kenneth J. Ruoff