On The Holy Trinity by St. Augustine
Chapter 9.--The Same Argument Continued.
He did not say, I and they are one thing; although, in that He is the head of the church which is His body, He might have said, and they are, not one thing, but one person, because the head and the body is one Christ; but in order to show His own Godhead consubstantial with the Father (for which reason He says in another place, |I and my Father are one| ), in His own kind, that is, in the consubstantial parity of the same nature, He wills His own to be one, but in Himself; since they could not be so in themselves, separated as they are one from another by divers pleasures and desires and uncleannesses of sin; whence they are cleansed through the Mediator, that they may be one in Him, not only through the same nature in which all become from mortal men equal to the angels, but also through the same will most harmoniously conspiring to the same blessedness, and fused in some way by the fire of charity into one spirit. For to this His words come, |That they may be one, even as we are one;| namely, that as the Father and Son are one, not only in equality of substance, but also in will, so those also may be one, between whom and God the Son is mediator, not only in that they are of the same nature, but also through the same union of love. And then He goes on thus to intimate the truth itself, that He is the Mediator, through whom we are reconciled to God, by saying, |I in them, and Thou in me, that they may be made perfect in one.|