Ambrose of Milan

Set before yourself any river. It springs from its fountain but is of one nature, of one brightness and beauty. And you assert rightly that the Holy Spirit is of one substance, brightness and glory with the Son of God and with God the Father. I will sum up everything in the oneness of the qualities without any dispute over degrees of greatness. For in this point also Scripture has provided for us. For the Son of God says, "Whoever shall drink of the water that I will give him, it shall become in him a well of water springing up unto everlasting life." This well is clearly the grace of the Spirit, a stream proceeding from the living Fountain. The Holy Spirit, then, is also the fountain of eternal life.You observe, then, from his words that the unity of the divine greatness is pointed out and that Christ cannot be denied to be a fountain even by heretics, since the Spirit, too, is called a fountain. The Spirit is called a river, too, just as the Father said, "Behold, I come down on you like a river of peace, and like a stream overflowing the glory of the Gentiles." And who can doubt that the Son of God is the river of life from whom the streams of eternal life flowed forth?