"God is in all things; not, indeed, as part of their essence, nor as an accident, but as an agent is present to that upon which it works. For an agent must be joined to that wherein it acts immediately and touch it by its power; hence it is proved in Phys. vii that the thing moved and the mover must be joined together. Now since God is very being by His own essence, created being must be His proper effect; as to ignite is the proper effect of fire. Now God causes this effect in things not only when they first begin to be, but as long as they are preserved in being; as light is caused in the air by the sun as long as the air remains illuminated. Therefore as long as a thing has being, God must be present to it, according to its mode of being. But being is innermost in each thing and most fundamentally inherent in all things since it is formal in respect of everything found in a thing, as was shown above. Hence it must be that God is in all things, and innermostly."
Aquinas, Summa Theologica
"Existence is contained in the idea or concept of every single thing, since we cannot conceive of anything except as existing. Possible or contingent existence is contained in the concept of a limited thing, whereas necessary and perfect existence is contained in the concept of a supremely perfect being."
Descartes, Third Meditation
"Prop. XV. Whatsoever is, is in God, and without God nothing can be, or be conceived."
"Prop. XX. The existence of God and his essence are one and the same."
"Whatever infinite or unconditional power and meaning are attributed to the highest being, it has ceased to be !a! being and has become being-itself. Many confusions in the doctrine of God and many apologetic weaknesses could be avoided if God were understood first of all as being-itself or as the ground of being. The power of being is to unite different traditions: the Augustinian, in which the divine existence is included in his essence, and the Aristotelian, which derives the existence of God from the existence of the world and which then asserts, in a second step, that his existence is identical with his essence. Thus the question of the existence of God can be neither asked nor answered. If asked, it is a question about that which by its very nature is above existence, and therefore the answer--whether negative or affirmative--implicitly denies the nature of God. It is as atheistic to affirm the existence of God as it is to deny it. God is being-itself, not !a! being."
Paul Tillich, Systematic Theology