Thomas Aquinas, Compendium of Theology

From all this it is evident that God exists always. For whatever necessarily exists, always exists; it is impossible for a thing that has no possibility of not being, not to be. Hence such a thing is never without existence. But it is necessary for God to be, as has been shown. Therefore God exists always.

Again, nothing begins to be or ceases to be except through motion or change. But God is absolutely immutable, as has been proved. Therefore it is impossible for Him ever to have begun to be or to cease to be.

Likewise, if anything that has not always existed begins to be, it needs some cause for its existence. Nothing brings itself forth from potency to act or from non-being to being. But God can have no cause of His being, since He is the first Being; a cause is prior to what is caused. Of necessity, therefore, God must always have existed.

Furthermore, whatever pertains to anyone in some other way than by reason of an external cause, pertains to him of himself. But existence does not come to God from any external cause, since such a cause would have to be prior to Him. Therefore God has existence of Himself, per se ipsum. But what exists per se exists always and necessarily. Therefore God exists always.