Novatian: Treatise Concerning the Trinity XXIX-XXXI
Moreover, the order of reason, and the authority of the faith in the disposition of the words and in the Scriptures of the Lord, admonish us after these things to believe also on the Holy Spirit, once promised to the Church, and in the appointed occasions of times given.So simple a summary of the faith regarding the Holy Spirit, and yet full and rich and applicable to the more-charismatic and less-charismatic alike. In the last section Novatian defends the doctrine and Deity of the Holy Spirit, with a simple interpretation that manages to contain much truth while avoiding the errors that some of his contemporaries (*cough* *cough* Tertullian) fell into. The Holy Spirit is the Person of the Trinity who is with us now, strengthening, supporting, and sanctifying.
And yet, to say that the Godhead is three persons is not to say that there are three gods. Novatian ends his treatise by defending the unity of God as One rather than as three divinities. The last chapter on the relationship within the Trinity makes me wish I knew Greek, because while it makes sense and has some depth in English, I'm sure the Greek words and phrases have a theological depth that may not quite be carried into English. Of course if it was written in Latin, then I've no excuse but my own laziness... [shrug]
Overall, this treatise is worth reading, although as I've said along the way part of it at least can be skimmed.