Until recently, there have been predominantly two ways of approaching Christian–Buddhist polemics during Japan's late medieval and early modern periods. One way is to look at the writings of European Jesuits. The other way is to look at the Buddhist response, largely seen in works produced by Zen monks. Until recently, we had yet to see an informed Japanese understanding of Christianity, nor an insider's critical treatment of Japanese Buddhism. In the person of Fukansai Habian, however, we see both of these things. Not only was he a convert to Christianity, but he was also a former Zen monk. We will examine his catechistic text, Myōtei Mondō (1605), and its recently discovered first fascicle on Buddhism as a nexus for considering Christianity's polemical challenge to Buddhism. We will see how the meeting of radically different worldviews regarding such fundamental issues as salvation, the afterlife, ethics, the ‘soul’, and the nature of Ultimate Reality/God came to a climax in the person of Habian and found its most thorough and informed exposition in Myōtei Mondō. Within this discussion, we will also address the Buddhist response to the charge that it is based on emptiness, nothingness, and void.