Ch 11 - "Identity of Essence and Existence in God" From the "Compendium of Theology" by St. Thomas Aquinas

Caput 11 Quod Dei essentia non est aliud quam suum esse

CHAPTER 11 IDENTITY OF ESSENCE AND EXISTENCE IN GOD

Ulterius autem necesse est quod Dei essentia non sit aliud quam esse ipsius. In quocumque enim aliud est essentia, et aliud esse eius, oportet quod aliud sit quod sit, et aliud quo aliquid sit: nam per esse suum de quolibet dicitur quod est, per essentiam vero suam de quolibet dicitur quid sit: unde et diffinitio significans essentiam, demonstrat quid est res. In Deo autem non est aliud quod est, et aliud quo aliquid est; cum non sit in eo compositio, ut ostensum est. Non est igitur ibi aliud eius essentia, quam suum esse.

God’s essence cannot be other than His existence. In any being whose essence is distinct from its existence, what it is must be distinct from that whereby it is. For in virtue of a thing’s existence we say that it is, and in virtue of its essence we say what it is. This is why a definition that signifies an essence manifests what a thing is. In God, however, there is no distinction between what He is and that whereby He is, since there is no composition in Him, as has been shown. Therefore God’s essence is nothing else than His existence.

Item. Ostensum est quod Deus est actus purus absque alicuius potentialitatis permixtione. Oportet igitur quod eius essentia sit ultimus actus: nam omnis actus qui est circa ultimum, est in potentia ad ultimum actum. Ultimus autem actus est ipsum esse. Cum enim omnis motus sit exitus de potentia in actum, oportet illud esse ultimum actum in quod tendit omnis motus: et cum motus naturalis in hoc tendat quod est naturaliter desideratum, oportet hoc esse ultimum actum quod omnia desiderant. Hoc autem est esse. Oportet igitur quod essentia divina, quae est actus purus et ultimus, sit ipsum esse. 

Likewise, we have proved that God is pure act without any admixture of potentiality. Accordingly His essence must be the ultimate act in Him; for any act that has a bearing on the ultimate act, is in potency to that ultimate act. But the ultimate act is existence itself, ipsum esse. For, since all motion is an issuing forth from potency to act, the ultimate act must be that toward which all motion tends; and since natural motion tends to what is naturally desired, the ultimate act must be that which all desire. This is existence. Consequently the divine essence, which is pure and ultimate act, must be existence itself, ipsum esse.