It was during its “Christian Century” (1549–1650) that Japan first encountered Western cosmology via the Jesuit missionaries. This paper explores how and why these missionaries used cosmology in their evangelical activities. My central argument is that the Jesuits considered cosmological knowledge a very effective “spiritual tool” in the first stage of their conversion plan: 1) to convince the catechumens on logical and rational grounds that God exists, then 2) to instill other Christian doctrines, and finally 3) to accept baptism. Much textual evidence shows that cosmological ideas were closely tied to references to “divine design”, which are found in treatises on catechism. This was a distinguishing characteristic of the early Jesuit mission in Japan.