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What did Thomas Aquinas mean by saying that God is pure being?

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Holbein the Younger: Boethius — Scholasticism Boethius, woodcut attributed to Holbein the Younger, 1537.

In the context of Scholasticism, St. Thomas Aquinas defined God as "pure being" or the "act of existing" (esse) to distinguish the Creator from all other creatures.12

For Aquinas, this concept highlights a fundamental metaphysical difference:
* Creatures: All finite beings possess a composition of essence and existence. They "participate" in being, meaning their existence is limited by their specific nature or essence (e.g., a human being exists only to the extent that their humanity permits).12
* God: In God, there is no such composition. God’s essence is his existence.12

By identifying God as pure being, Aquinas posits that God is not merely a being among others, but the unconditioned, infinite source of existence itself. While finite things are dependent on external causes for their act of existing, God is necessary in himself, serving as the ultimate foundation for all that exists. This understanding allowed Aquinas to harmonize Aristotelian philosophy with Christian theology, framing God as the absolute, unchanging act of being.3412