In 1855 Parley P. Pratt, a Mormon missionary and member of the Quorum of the Twelve, published Key to the Science of Theology. It was the culmination of over twenty years of intellectual engagement with the young religious movement of Mormonism. The book was also the first attempt by any Mormon at writing a comprehensive summary of the religion's theological ideas. Pratt covered topics ranging from the origins of theology in ancient Judaism, the apostasy of early Christianity, the restoration of correct theology with nineteenth century Mormonism, dreams, polygamy, and communication with beings on other planets. For nearly fifty years after its publication, Key to the Science of Theology was one of the most widely circulated books within the Mormon community, serving as a model of doctrinal orthodoxy. This thesis aims to understand Pratt's book and his theological ideas, broadly, in their historical context. Primary sources related to Pratt and his contemporaries, including other works by Pratt, Mormon missionary tracts, newspaper clippings, and theological writings by competing religions, help place Pratt's ideas within the larger framework of American religious and intellectual thought of the early to mid-nineteenth century. Pratt drew from non-Mormon sources to help explain the Church's teachings, at times appropriating ideas and rhetoric from elsewhere to bolster his claims about the superiority and universality of the Mormon message. The first chapter of this thesis gives a biographical sketch of Pratt. It introduces key concepts in Mormon belief and how Pratt conceived them. Furthermore, the chapter offers a philosophical take on Pratt's life as one motivated by an apocalyptic worldview. Chapter two draws upon Pratt's apocalyptic conscience to examine his eschatological ideas including a strain of early Mormon thought regarding theocracy. Pratt envisioned a world-wide theocracy coming at the millennium. Mormons, Jews, and Native Americans as ancient Israelites would all share in a world-wide order built around twin centers of power in the historical Jerusalem and a New Jerusalem to be established in North America. Chapter three looks at Pratt's cosmology and argues that his views of the universe, including other planets and beings, were influenced and framed by contemporary Spiritualism as a means of combatting the threat of Mormons leaving the Church for Spiritualist practices. The epilogue looks at changes made to the text of Key to the Science of Theology in 1915 by Church leader Charles Penrose. It places the text's republication within an ongoing battle between older Church leaders like Penrose and younger leaders such as John Widtsoe over what would constitute Mormon orthodoxy during the modernizing phase of the Church in the early twentieth century. Issues like evolution and polygamy took the forefront over eschatological and cosmological concerns.