Thesis advisor: Boyd T. Coolman / Achard of St. Victor's (1100-1171) theology is best understood through the lens of participation in God. He identifies three modes of participation: creation, righteousness, and beatitude. Participation by creation denotes the common image of God found in all humans. Participation by righteousness is the central focus of Achard's theology and consists of the increase of virtue, manifest in the love of God and neighbor. Finally, participation by beatitude is unity Trinity. The modes of participation are progressive, each on building upon the previous mode. Participation establishes a framework which situates Achard's Christology, pneumatology, Trinitarian theology, theological anthropology, and ethics and also creates a theology that takes an individual's virtue as the starting point. This participation framework bridges speculative theology and practical application, reflecting the ecclesiastical reform movements of his time. The result is theology of Christian life that is a balance between contemplation and concrete action. Achard expresses his participation centered theology through the use of homiletical images that serve to teach and inspire. I argue that Achard has a master symbol of a triple interior cathedral that is built by Christ, through grace, in the souls of the faithful. The building of this structure corresponds with progress in the spiritual life, moving from participating in God through creation, righteousness, and beatitude. Achard's theology presents a dynamic relationship between theological doctrines and images, between pedagogy and application, and between the present life and the life to come. / Thesis (PhD) — Boston College, 2015. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Theology.