This is a thorough study of the influence of Platonism, Neoplatonism, and Christianity on Willa Cather's writing. It aims to show how Cather used Platonic-Plotinian and Christian ideas and images within her fiction and nonfiction. The ideas of Plato's dialogues and the writings of Neoplatonists (who were more popular in the West than Plato in the early Middle Ages) were assimilated into, as described in St. Augustine's Confessions and The City of God, a Christian culture by the Church Fathers. However, St Augustine, highly influenced by the Neoplatonic ladder of love, reconstructed it to suit his Christian worldview. Platonic pride and the cult of the heroic personality who longs for self-development and self-realization were substituted for the concept of God's grace: only by surrendering to God's will, relying on God's grace and surpassing his or her pride and self-centeredness, can a genuine believer reveal God's presence in his life. It is evinced that Cather's early writings (The Kingdom of Art, The Troll Garden, and some early short stories, in particular "The Joy of Nelly Deane") are permeated with such Platonic ideas and images as symbolism of the light (which points to the illumination and awakening of the soul), the cicadas myth, the idea of immortality of the soul, and the Platonic-Plotinian theory of ecstatic creativity (to which Cather adds the necessity of learning an artistic tradition as well as vocational craft). To Cather, only a combination of God's gift, knowledge of a vocational technique, and unremitting and dedicated labor can produce genius in art. Notwithstanding Cather's favorite characters' yearning for the ideal world (i.e. the Platonic world of the True, the Beautiful, and the Good), the terrestrial world, and life in general, are never totally discarded in her works. This suggests that Cather's admiration for the Platonic ascetic ideal is leveled by her simultaneous desire to worship life and nature. Such a reverence for life and nature must be indebted to Plotinus's theory of emanations, which aimed to bridge the gap between the terrestrial and celestial worlds and was expanded upon by the German idealists Shelling and Fichte, not to mention Carlyle and Emerson. Overall, Cather does not differentiate between Plato and Plotinus, considering them as one Platonic-Neoplatonic tradition, perhaps because Plato has been read through a Neoplatonic lens for centuries. In Cather's early works, a pull towards Christian imagery is also discernible, but it appears as an undertone suggesting that the Platonic cult of beauty, enhanced by the Neoplatonic cult of art (which was evolved by romantics, aesthetes, and symbolists -- through whose writings Cather also could become interested in the Platonic tradition), are the predominant images within these works. Cather's religious preferences become more apparent after World War I when she begins to depict Christianity, and especially the Roman Catholic Church, with a great amount of veneration and admiration. She seems to be perpetually drawing on Christian imagery and alluding to both the Bible as well as philosophers and theologians like St. Augustine and Pascal. In addition, Cather employs the narrative modes of hagiography and legends (the legends of the Virgin Mary apparitions, in particular). In her later novels, A Lost Lady, Death Comes for the Archbishop, and Shadows on the Rock, which are analyzed in this study, Cather presents Christianity as a powerful, unifying, and illuminating force, a counterforce against the powers of disorder and disunity. Yet, it is also worth mentioning that the Platonic- Plotinian cult of beauty and art is also apparent in Cather's later novels, insofar as the aesthetic side of Roman Catholicism is constantly highlighted through the depiction of beautiful sacramental rituals of the church and religious images of the saints and Mary. Thus in these works the Platonic and Christian traditions intermingle. While in Cather's early fiction and nonfiction the religion of art and beauty is promoted (thereby betraying her aestheticism and elitism), her later novels convey the idea of the indivisibility of art and religion -- an idea that was widespread in the Middle Ages, but had been put into question since the Renaissance, a movement which caused art to become more and more secular and autonomous. In Cather's later works, the medieval atmosphere of miracles, the chivalric ideal of worshipping the Beautiful Dame, and the Christian utopia of the Heavenly City strongly indicate that the Middle Ages was the historical period that Cather most revered, as evinced by her repeated projections of the Middle Ages on much later time periods within world history. The early Christian church is presented as an example to be emulated -- a sacred union, which sought to conform to Jesus' ideal of love and brotherhood. In these novels, Cather's favorite characters aim to restore and strengthen faith, and to bring the cross to remote lands of the New World, claiming these lands for God and joining (through the sacrament of baptism) their inhabitants to the long and glorious Christian tradition.