Although western medieval thinkers in general had a little interest in non-Christian religious traditions, one can find a sort of comparative study of religions already in the 13th century. As a rule, the purpose of such studies was to prove the superiority of Christianity over other religions, and the means used were often the instruments of ‘natural human reason’. Works of Roger Bacon (1214/22–ca. 1292) is the most striking example of this approach. Bacon does not appeal to the text of the Bible while he compares one religion with another on the basis of their compliance with the requirements of ‘moral philosophy’ (Bacon’s original version of natural theology). The article reconstructs three Bacon’s classifications of religions, describes their place with- in the framework of his ‘moral philosophy’, and shows their relationship with his scientific ideas.