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How did Aquinas’s understanding of God differ from Aristotle’s prime mover?

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St. Thomas Aquinas Enthroned Between the Doctors of the Old and New Testaments, with Personifications of the Virtues, Sciences, and Liberal Arts, fresco by Andrea da Firenze, c. 1365; in the Spanish Chapel of the Church of Santa Maria Novella, Florence. — Thomism *St. Thomas Aquinas Enthroned Between the Doctors of the Old and New Testaments, with Personifications of the Virtues, Sciences, and Liberal Arts*, fresco by Andrea da Firenze, c. 1365; in the Spanish Chapel of the Church of Santa Maria Novella, Florence.

While Thomas Aquinas adopted Aristotle’s philosophical framework—including the proof of a prime mover—to demonstrate the existence of God, he fundamentally transformed the concept to align with Christian and Jewish theology.123

The primary difference lies in the nature of being. Aquinas established a harmony between Aristotelianism and faith by introducing a new understanding of existence (esse). For Aquinas, God is "pure being," or the act of existing itself, where God’s essence is identical to his existence. In contrast, creatures are composed of both essence and existence, meaning they only participate in being to the extent their nature allows.12

Furthermore, while Aristotle’s prime mover served as a philosophical necessity to explain motion and causality, Aquinas identified this source as the personal God of revelation. By modifying Aristotelian principles, Aquinas ensured that his theological system never compromised Christian doctrine, instead correcting or deepening philosophical concepts whenever they clashed with faith.132