In Paradise Lost, John Milton's language for the heart reflects his monist belief in the inseparability of body and soul. As the voices within the epic speak with and through each other, the heart is characterised as the centre of the individual. This thesis reads Milton's view of the heart in the light of his theological context, particularly the various translations of the Bible available to him, beginning with that of William Tyndale, the first translation of the Bible directly from the original languages into English. Milton's figurative language and style are rooted in the scriptures, in which the New Testament interprets the Old. A New Testament book that is particularly illustrative of the revision of the Old Testament is the epistle to the Hebrews, which is an important biblical place in relation to Paradise Lost. Other intertexts include Tyndale's commentaries, the Authorised Version of the Bible, and Milton's own theological treatise, On Christian Doctrine. The first section of the thesis considers the place of the heart in the world, where it functions as a commonplace book, gathering and keeping the word. The second section examines Milton's language for hardness or fleshiness of heart. The final section considers the heart in worship in an argument that sees Milton entering into the theological debates of his day as he negotiates a place for outward religious rites and makes the heart the most important place of worship.