Thomas Aquinas was a profoundly influential thinker from the thirteenth century. As a scholastic, Aquinas sought to understand Christian theology in light of the rediscovery of Aristotle’s works, and he redefined the relationship between revelation and reason, science and theology, and faith and philosophy for the next eight centuries. As a philosopher, Aquinas developed principles of just war and natural law, and outlined an argument for God’s existence from contingency—the intellectual forerunner to the modern Argument from Design.
During the Reformation, Aquinas’ influence waned. Calvin and Luther rarely interacted with his works, preferring Augustine and the Early Church Fathers. The Catholic Church still held his works in high regard, but other scholastics, such as Duns Scotus, were more influential in the Catholic Counter-Reformation. It wasn’t until the nineteenth century that Aquinas was elevated to his current status in the Catholic Church.
He also received renewed interest in Protestant circles as well. In the early nineteenth century, Herman Bavinck interacts with Aquinas a great deal in Reformed Dogmatics, mostly in his volume on the doctrine of God. In fact, Bavinck cites Aquinas 354 times in his 4-volume work. More recently, Norman Geisler has mentioned that Aquinas’ Summa Theologica is his favorite book after the Bible.
This isn’t to say Aquinas was a proto-Protestant. At the same time, it’s almost impossible—in any Christian tradition—to have a conversation about God’s attributes, simplicity, knowability, or any number of other topics without interacting with Aquinas.