Thomas Aquinas argues the existence of God on the basis of potentially and actuality. For example, an essentially ordered series is a sequence in which each member of that sequence relies on another member to complete an action. Such as, if you have a stone and a stick, the stick pushes the stone, but what is moving the stick? Your hand. Without the first member of the series the last member, the moving of the stone, could not exist which leads us back to the hand, the hand that is not the first member of the series.
God is the first member of the series, he cannot change or move like the hand that could potentially be cut off. We exist because God exists, God made our bodies which uses nerves to move muscles to move the arm which moves the hand and then the stick and stone. God is pure act, having no potential of changing or moving.
Some later Scholastics, Jon Duns Scotus and William of Ockham argued that Aquinas’ theory of the existence of God is incorrect. Aquinas uses reason with the example of the stick and stone to prove God’s existence but these Scholastics challenge him because they believe the existence of God is only knowable by faith, faith that is totally separate from reason.