This book proposes that Christian worship is a key source for any theology seeking to understand the covenant between God and human beings in the Christian tradition. Through a detailed examination of phenomenological, biblical and theological sources, the author seeks to write a theology in which the selfhood of both God and human beings is seen as essentially 'vowed' or 'covenantal.' This claim is then explored through a detailed examination of Eucharistic worship, which is understood as a 'non-identical performance' of the covenant established between God and human beings in baptism. Here, then, is a theology that understands Christian worship not simply as 'form' or 'event' but, more radically, as a mutual act of promising and commitment between God and human beings.