On The Holy Trinity by St. Augustine
Chapter 2.--God the Only Unchangeable Essence.
3. He is, however, without doubt, a substance, or, if it be better so to call it, an essence, which the Greeks call ousia. For as wisdom is so called from the being wise, and knowledge from knowing; so from being comes that which we call essence. And who is there that is, more than He who said to His servant Moses, |I am that I am;| and, |Thus shall thou say unto the children of Israel, He who is hath sent me unto you?| But other things that are called essences or substances admit of accidents, whereby a change, whether great or small, is produced in them. But there can be no accident of this kind in respect to God; and therefore He who is God is the only unchangeable substance or essence, to whom certainly being itself, whence comes the name of essence, most especially and most truly belongs. For that which is changed does not retain its own being; and that which can be changed, although it be not actually changed, is able not to be that which it had been; and hence that which not only is not changed, but also cannot at all be changed, alone falls most truly, without difficulty or hesitation, under the category of being.