Despite being the first extended defense of the divinity of the Holy Spirit written in Latin and influencing the Trinitarian theology of Augustine of Hippo, Ambrose of Milan’s On the Holy Spirit (De Spiritu Sancto) has received little scholarly attention. This book seeks to change this perspective by claiming that Ambrose defines the Holy Spirit in a way consistent with pro-Nicene theology using classical Ciceronian rhetoric to interpret Scripture in a quasi-judicial situation.