Shigyaku Keiyakusha Fausts

Tamiya Rokuto and Endou Hiroto are both child prodigies in the mathematics world, but they are both terminally ill. These two stay at a special hospital that is being run by a Christian group, where nuns act as nurses and priests act as doctors. At this hospital, all of the patients are highly respected academics in their own field. These individuals have been gathered to decode a special book that allegedly contains the secrets to immortality. To save Endou's life, Tamiya sets out to decode this mysterious book, but there seems to be demonic forces that come with this forbidden book.

(Source: MangaHelpers)