St. Thomas Aquinas(1225-1274), was a very influential medieval philosopher, who is regarded as one of the greatest theologians of the catholic church. Because he was a philosopher he is best known for his use of reason in his work. One of the ways he used reason was by deriving the divine attributes of God. Two of these divine attributes that Aquinas derived were: immaterial and all-powerful.
Being material means that a thing must be able to change or move. But a purely actual being cannot be changeable or moveable; also a purely actual being is outside time and space, which is the opposite of the material thing, which is inside time and space. So basically God doesn’t change, and is outside of time and space, unlike the material thing. Along with this point, since God controls everything, and cannot be controlled by anything, he is the unmoved mover.
The divine attribute of all-powerful is attributed to God for this main reason: each material thing has something that they can or cannot do. But God, because God is the source of all changes of things coming to have the attributes they have, He possesses all of these qualities, hence why He is all-powerful.
Aquinas as a philosopher believed that there was a way to explain the existence of God through reason. The way that he derived the divine attributes including the ones that were explained, was by noting that God is pure act.