So let us believe also in the Holy Spirit. He is God, you see, because it is written, “God is Spirit.” Through him we have received the forgiveness of sins, through him we believe in the resurrection of the flesh, through him we hope for life everlasting. But take care you don’t fall into error by counting, and imagine I have said there are three gods, because I have given the one God a third name. There is one substance of godhead in the Trinity, one power, one might, one majesty, one name of divinity.

—Augustine, Sermon 215.8