The teachings of St. Feofan (1815-1894), together with his celebrated Russian translation of the Philokalia, played a major role in the spiritual revival of nineteenth century Russia, earning him the reputation of one of the greatest Russian theoreticians of Orthodox mysticism. Yet Feofan also provoked fierce criticism of such eminent Russian thinkers as Nicolas Berdyaev. This study focuses on one of the most significant elements of Feofan's legacy: the notion of legitimacy of spiritual diversity. This idea permeates Feofan's discussions of the striking dissimilarities demonstrated by persons engaged in ascetic and spiritual pursuits. It is highlighted by the fact that Feofan derives his positive attitude to diversity from within the tradition generally perceived as intolerant of dissent and non-conformity. Based within a wider context of Orthodox mystical tradition, this study examines Feofan's major writings, his work with the texts of the Philokalia, and his public and private correspondence. Attention is given to such pivotal concepts as his notions of the spirit and the heart; intimate communion with God; and the cardinal importance of spiritual self-consciousness. The study reveals Feofan's indebtedness to Byzantine ascetic spirituality, as well as his unusual openness to Western thought - reflected in his innovative synthesis of patristic asceticism and German romantic psychology. This study also addresses the ambivalence of Feofan's inner conflicts: namely, between his passionate belief in spiritual self-determination and his authoritarian teaching on 'silent submissiveness'; and between his defence of the legitimacy of spiritual diversity within the realm of ascetic experience and his unwillingness to extend that same principle beyond the limits of his own tradition. The thesis is not only the first critical account in English of Feofan's spiritual teaching, but is also an ecumenical exercise in which attitudes toward religious diversity within the Russian monastic tradition are examined by a Russian evangelical.