Exod 15:18

“That the divine being is eternal, the authority of Scripture demonstrates this, Exodus 15: The Lord shall reign forever and ever; and Romans 1: His eternal power also and divinity. The truth of the faith also demonstrates this: "Eternal the Father," it says in the Athanasian Creed, "eternal the Son, eternal the Holy Spirit." Likewise, the necessity of reason concludes this same thing. For everything that is its own being is eternal; for being cannot not be, therefore it can neither begin nor cease, and thus it lacks a beginning and an end: if therefore God, since he is most simple, is his own being, indeed is simply being itself; therefore he is altogether eternal.”