This dissertation is a study of the strikingly similar prayers found in chapters 27 and 50 of the Acts of Thomas. These distinctive prayers give evidence of an understanding of Spirit that did not survive in the dominant Christian church. The prayers in question address the Spirit, being feminine in Syriac (probably the original language of the work), as 'Mother,' 'fellowship of the male,' and 'dove.' The epicleses appeal to the Spirit with a series of appellations describing her as one who reveals hidden mysteries, who is a compassionate Mother, and who provides joy and rest to her adherents. The prayers ask her to 'come' and be present in the ritual actions of initiatory anointing and Eucharist. The dissertation seeks to place these prayers within their historical, literary, and liturgical contexts. The composite nature of the Acts of Thomas is emphasized, and it is suggested that the author of the unified second half of the work also edited the discrete tales that appear in the first half. In contrast to earlier scholarship, this study suggests that Acts of Thomas stems from the middle of the third century and has been brought together in its completed form by someone with ties to the northern Mesopotamian city of Nisibis. The redactor has inserted prayers and speeches into the narrative; some of the prayers were apparently composed for the occasion, but others, including the epicleses of interest to the study, were most likely in use in and adapted from liturgical contexts. The Acts of Thomas includes several initiation rituals, including those in which the epicleses appear. Although there is variety in the rituals, they point to a prominence accorded the oil of anointing, to a Eucharist of bread and water, and to a practice of water baptism that is tertiary in significance, if present at all. Finally, the dissertation compares the epicleses in form and content with other prayers from the ancient world, and discusses their emphasis on a revelatory figure who is present in ritual action and who provides her adherents with a glimpse of an otherworldly reality.