Objection 1. It seems that God is not the same as His essence or nature. For nothing can be in itself. But the substance or nature of God--i.e. the Godhead--is said to be in God. Therefore it seems that God is not the same as His essence or nature.
Objection 2. Further, the effect is assimilated to its cause; for every agent produces its like. But in created things the "suppositum" is not identical with its nature; for a man is not the same as his humanity. Therefore God is not the same as His Godhead.
On the contrary, It is said of God that He is life itself, and not only that He is a living thing: "I am the way, the truth, and the life" (John 14:6). Now the relation between Godhead and God is the same as the relation between life and a living thing. Therefore God is His very Godhead.
I answer that, God is the same as His essence or nature. To understand this, it must be noted that in things composed of matter and form, the nature or essence must differ from the "suppositum," because the essence or nature connotes only what is included in the definition of the species; as, humanity connotes all that is included in the definition of man, for it is by this that man is man, and it is this that humanity signifies, that, namely, whereby man is man. Now individual matter, with all the individualizing accidents, is not included in the definition of the species. For this particular flesh, these bones, this blackness or whiteness, etc., are not included in the definition of a man. Therefore this flesh, these bones, and the accidental qualities distinguishing this particular matter, are not included in humanity; and yet they are included in the thing which is man. Hence the thing which is a man has something more in it than has humanity. Consequently humanity and a man are not wholly identical; but humanity is taken to mean the formal part of a man, because the principles whereby a thing is defined are regarded as the formal constituent in regard to the individualizing matter. On the other hand, in things not composed of matter and form, in which individualization is not due to individual matter--that is to say, to "this" matter--the very forms being individualized of themselves--it is necessary the forms themselves should be subsisting "supposita." Therefore "suppositum" and nature in them are identified. Since God then is not composed of matter and form, He must be His own Godhead, His own Life, and whatever else is thus predicated of Him.
Reply to Objection 1. We can speak of simple things only as though they were like the composite things from which we derive our knowledge. Therefore in speaking of God, we use concrete nouns to signify His subsistence, because with us only those things subsist which are composite; and we use abstract nouns to signify His simplicity. In saying therefore that Godhead, or life, or the like are in God, we indicate the composite way in which our intellect understands, but not that there is any composition in God.
Reply to Objection 2. The effects of God do not imitate Him perfectly, but only as far as they are able; and the imitation is here defective, precisely because what is simple and one, can only be represented by divers things; consequently, composition is accidental to them, and therefore, in them "suppositum" is not the same as nature.
Source: Summa Theologica (New Advent)