God’s Essence is His Existence

...if the existence of a thing differs from its essence, this existence must be caused either by some exterior agent or by its essential principles. Now it is impossible for a thing’s existence to be caused by its essential constituent principles, for nothing can be the sufficient cause of its own existence, if its existence is caused. Therefore that thing, whose existence differs from its essence, must have its existence caused by another. But this cannot be true of God; because we call God the first efficient cause. Therefore it is impossible that in God His existence should differ from His essence.

Si igitur ipsum esse rei sit aliud ab ejus essentia, necesse est, quod esse illius rei vel sit causatum ab aliquo exteriori, vel a principiis essentialibus ejusdem rei. Impossibile est autem, quod esse sit causatum tantum ex principiis essentialibus rei, quia nulla res sufficit, quod sit sibi causa essendi, si habeat esse causatum. Oportet ergo, quod illud, cujus esse est aliud ab essentia sua, habeat esse causatum ab alio; hoc autem non potest dici de Deo, quia Deum dicimus esse primam causam efficientem; impossibile est ergo, quod in Deo sit aliud esse, et aliud ejus essentia.


Thomas Aquinas, Summa Theologia, 1.3.4