On The Holy Trinity by St. Augustine
Proceeds to treat of the arguments put forward by the heretics, not from Scripture, but from their own reason. Those are refuted, who think the substance of the Father and of the Son to be not the same, because everything predicated of God is, in their opinion, predicated of Him according to substance; and therefore it follows, that to beget and to be begotten, or to be begotten and unbegotten, being diverse, are diverse substances; whereas it is here demonstrated that not everything predicated of God is predicated according to substance, in such manner as He is called good and great according to substance, or anything else that is predicated of Him in respect to Himself; but that some things are also predicated of Him relatively, i.e. not in respect to Himself, but to something not Himself, as He is called Father in respect to the Son, and Lord in respect to the creature that serveth Him; in which case, if anything thus predicated relatively, i.e. in respect to something not Himself, is even predicated as happening in time, as e.g. |Lord, thou hast become our refuge,| yet nothing happens to God so as to work a change in Him, but He Himself remains absolutely unchangeable in His own nature or essence.