E1P20D
Demonstration — Part I
Latin
Deus (per antecedentem propositionem) ejusque omnia attributa sunt æterna hoc est (per definitionem 8) unumquodque ejus attributorum existentiam exprimit. Eadem ergo Dei attributa quæ (per definitionem 4) Dei æternam essentiam explicant, ejus simul æternam existentiam explicant hoc est illud ipsum quod essentiam Dei constituit, constituit simul ipsius existentiam adeoque hæc et ipsius essentia unum et idem sunt. Q.E.D.
English (Elwes 1883)
God (by the last Prop.) and all his attributes are eternal, that is (by Def. viii.) each of his attributes expresses existence. Therefore the same attributes of God which explain his eternal essence, explain at the same time his eternal existence--in other words, that which constitutes God's essence constitutes at the same time his existence. Wherefore God's existence and God's essence are one and the same. Q.E.D.
Coroll. I.--Hence it follows that God's existence, like his essence, is an eternal truth.
Coroll. II--Secondly, it follows that God, and all the attributes of God, are unchangeable. For if they could be changed in respect to existence, they must also be able to be changed in respect to essence--that is, obviously, be changed from true to false, which is absurd.
Modern English
God and all his attributes are eternal (E1P19), that is, (E1D8), each of his attributes expresses existence. The same attributes of God that (E1D4) explain his eternal essence therefore also explain his eternal existence: the very thing that constitutes God's essence also constitutes his existence. So his existence and his essence are one and the same. Q.E.D.