What did St. Thomas Aquinas mean by describing God as pure being?

For St. Thomas Aquinas, describing God as "pure being" or the "act of existing" (esse) is central to his distinction between God and all other creatures.12

Aquinas argued that while all created things possess a composition of essence (what they are) and existence (the fact that they are), God is unique because his essence is his existence. In this view, creatures only "participate" in being; they receive existence to the extent permitted by their specific nature or essence.12

Conversely, God does not merely possess existence as a quality; he is existence itself. This concept serves as the fundamental ontological distinction in Aquinas’s philosophy, separating the Creator, who is necessary and self-existent, from all contingent beings that rely on an external cause for their existence. By identifying God as the act of existing, Aquinas synthesized Aristotelian principles with Christian theology, framing God as the ultimate source of all reality.312