How did St. Thomas Aquinas distinguish God from created beings?

St. Thomas Aquinas distinguished God from created beings primarily through his understanding of being as the act of existing (esse).12

For Aquinas, God is defined as pure being, meaning that in God, essence and existence are identical; God’s essence is his existence. In contrast, all created beings possess a real composition of essence and existence. Creatures participate in the act of existing only to the extent permitted by their specific essence—for example, human beings participate in being according to the limits of their humanity.12

Consequently, Aquinas posited that existence is distinct from essence only in created beings, whereas in God, they are one and the same. This fundamental ontological distinction underscores his view that the creative act is reserved to God alone, as creatures are dependent participants in the act of being rather than being itself.3