This book is a study of the historical development and impact of John Calvin’s doctrine of baptismal efficacy. The primary questions it addresses are (1) whether Calvin taught an “instrumental” doctrine of baptism, according to which the external sign of the sacrament serves as a means or instrument to convey the spiritual realities it signifies, and (2) whether Calvin’s teaching on baptismal efficacy remained constant throughout his lifetime or underwent significant change. Secondarily, the work also examines whether such spiritual blessings, in Calvin’s view, are conferred only in adult (believer) baptism or also in the baptism of infants, and what impact Calvin’s doctrine of baptismal efficacy had on the Reformed confessional tradition that followed him. The book examines all of Calvin’s writings on baptism—his Institutes, commentaries on Scripture, catechisms, polemical writings, and consensus documents—chronologically through five stages of his life and then analyzes the doctrine of baptismal efficacy in eight of the major Reformed confessions and catechisms from the age of confessional codification. It concludes that Calvin did indeed hold to an instrumental view of baptism; that this doctrine underwent change and development over the course of his life but not to the extent that some in the past have suggested; that his view of the efficacy of infant baptism was consistent with his doctrine of baptism in general; and that versions of Calvin’s teaching can be found in many, though not all, of the major Reformed confessional documents of the sixteenth and seventeenth centuries.