This study presents Gregory the Great as a theologian, in the sense of his having a cohesive body of teaching, even a system, although not systematically expressed. I draw out this system by studying Gregory's concept of "sacrifice," a point of convergence that theologically synthesizes his views on God, the created world, the human person, the Incarnation, the Church, and the Christian journey to full participation in the divine life. "Sacrifice," as a way into Gregory's thought, demonstrates the propriety of calling him a "theologian." In the long-running late antique debate on the nature of Christian "perfection," Gregory's sacrificial theology of human life with God makes a speculative contribution: a new theory of perfect love that makes possible an aspiration to contemplative perfection even in worldly life. This achievement illuminates the ever-murky questions of Gregory's relationships to predecessors and his legacy for the future. His teaching on sacrifice accomplishes a theological re-negotiation of the clashing traditions of Augustine and Cassian, opening a broad horizon of new cultural possibilities by elaborating a theory of spiritual perfection capacious enough to embrace the secular.