Fabian Fucan was born in the Kyoto region of Japan around 1565. He initially entered a Buddhist monastery, but in 1583 he converted to Catholicism and became a dojuku (a catechist affiliated with the Society of Jesus) the following year. In 1586, he was admitted into the Jesuit order as an iruman (lay brother), collaborating with missionaries to translate European texts into Japanese, preaching publicly, and engaging in debates with Buddhists and Neo-Confucians.

In 1605, he wrote Myotei Mondo (The Dialogue Between Myoshu and Hyu), a work in which he introduced Christianity to the Japanese public through a conversation between two noblewomen, while refuting other religions and doctrines present in Japan, namely Shintoism, Buddhism, and Confucianism. The Jesuit influence on his education is evident in this text: Fucan introduces several concepts unfamiliar to Japanese thought but central to Western philosophy, such as the rational soul, God as the efficient cause, the notion of substance, and the concept of matter.

He also makes explicit references to the Japanese context. For example, he explains that the commandment to worship one God implicitly forbids the veneration of kami, which he describes as manifestations of the devil. Regarding the commandment to honor one’s father and mother, he invokes the concept of filial piety, a value deeply rooted in Japanese culture.

Fucan also defended European Jesuits against accusations of acting as agents of foreign powers and of inciting discord among the Japanese. On the contrary, he argued, Christians were loyal subjects of the Emperor and the Shogun, and that the Christianization of Japan would bring prosperity and harmony.

Despite his aspirations, the Jesuits never admitted Fucan to the priesthood, possibly due to a disastrous debate he had with a Neo-Confucian scholar, which may have caused them to reconsider his abilities. As a result, Fucan left the Society. Some years later, he reemerged as a collaborator with Japanese authorities during the Tokugawa shogunate's anti-Christian persecutions. It was not uncommon for officials to use Japanese apostates to gather information and identify Christians.

Fucan did not merely renounce his faith and align himself with the persecutors, he went further. In 1620, he authored a work titled Ha Daisu (Against the Christian God), in which he denounced the Christian doctrine and its missionaries as false and dangerous. He particularly accused the European Jesuits of racism, claiming they refused to ordain Japanese into the priesthood.