In the second half of the fourth century the mystery of the Holy Spirit was the subject of fierce debate. Those who fought against the Nicene Creed opposed the idea that the Spirit was God. Even some of those willing to accept the equality of the Father and the Son saw the Spirit as more angelic than divine. The first great testament to the Spirit's divinity -showing how the Spirit creates and saves inseparably with the Father and the son- is St. Athanasius' Letters to Serapion. Only a few years later, Didymus the Blind penned his own On the Holy Spirit, which is here translated into English for the first time. For Didymus, the Spirit transforms Christians by drawing them into the divine life itself, and must therefore be one with the Father and Son. This volume offers new translations of two of the most powerful Patristic reflections on the work and nature of the Holy Spirit.