“It is not easy to find a name that will suitably express so great an excellence, unless it is better to speak in this way: the Trinity, one God, of whom are all things, through whom are all things, in whom are all things.
Thus the Father and the Son and the Holy Spirit, and each of these by Himself, is God, and at the same time they are all one God; and each of them by Himself is a complete substance, and yet they are all one substance.
The Father is not the Son nor the Holy Spirit; the Son is not the Father nor the Holy Spirit; the Holy Spirit is not the Father nor the Son: but the Father is only Father, the Son is only Son, and the Holy Spirit is only Holy Spirit.
To all three belong the same eternity, the same unchangeableness, the same majesty, the same power. In the Father is unity, in the Son equality, in the Holy Spirit the harmony of unity and equality.
And these three attributes are all one because of the Father, all equal because of the Son, and all harmonious because of the Holy Spirit.”
–Augustine of Hippo, On Christian Doctrine, I.V.5.