ELI5: Why is St. Thomas Aquinas the definitive figure in Catholic Theology and Philosophy?

I hear about St. Thomas Aquinas often, as we all do. Sometimes he’s referred to as “St. Thomas” as if he’s replaced the apostle. Can someone explain why the ideas of this one man have become the leading philosophy within Catholicism? I struggle to understand how a single man who isn’t Christ has steered the course of history with his ideas.