This follows from His being the Purely Actual, Uncaused, First, Absolute, Most Intelligent Being.
Benedict Ashley has a nice summary of the proof from motion (see Q2 A3) which allows us to establish, from empirical evidence, God's simplicity as Pure Act:
For a body to exist in motion, it must be moved by another actual mover. This actual mover is either:
(a) Itself an existent body that acts only because it too is being moved by another, or
(b) An existent mover that is not a body.
But an infinite series of movers such as posited in (a) is impossible because they would be only potential not actual movers.
Therefore, since bodies in motion are observed to exist, a First Unmoved Mover that is not a body must exist, and such an entity is what is meant in ordinary usage by the word “God”.