Why did theologians like Thomas Aquinas care about proving God's existence? Wasn't Atheism nonexistent at that point? What were they trying to prove and to who?

by cyberpunkbeavis
Herissony_DSCH5

A very similar question was answered three months ago:

Why did St. Thomas Aquinas, in Summa Theologica, write about arguments for the existence of God and Christianity if everybody back then in Medieval Europe was Christian theist? Who was his intended audience?