"On the Holy Spirit"
by St. Ambrose

St. Ambrose’s treatise On the Holy Spirit is a sequel to On the Faith, and the two together are sometimes referred to as De Trinitate.  In the first book St. Ambrose proves that the Holy Spirit is truly God by showing how the Holy Spirit is in Scripture called the Spirit of God and that He spoke by the prophets and apostles, that He sanctifies men, and is typified by the mystical ointment spoken of in Scripture.  Then Ambrose sets about showing how the Holy Spirit is one with the other two Persons of the Holy Trinity.  The second book shows that the power of the Holy Spirit is the same as that of the Father and the Son and that He is Creator and therefore to be worshipped.  The third book continues the same argument, showing that the mission of prophets and apostles is to referred to by the Spirit. 

Source Used:  Nicene and Post-Nicene Fathers, Second SeriesVol. 10.  Translated by H. de Romestin, E. de Romestin and H.T.F. Duckworth.  Edited by Philip Schaff and Henry Wace. (Buffalo, NY: Christian Literature Publishing Co., 1896.)