Needless to say, Aquinas upset many of the popular theological ideas prevalent before him. Even though his work was unfinished at the time of his death, his ideas were brought into the theology of the church, giving Christianity a genuine intellectual and rational foundation. Aquinas' work influenced the philosophical climate of the day and gave reason a legitimate place in Christian theology.
One of Thomas's most important proofs is based on the idea that all movement has a cause. For a body to move, there must be something to cause the movement. Obviously, there is an abundance of movement in the world. According to Aquinas, it is clear that some being must have initiated the first movement. It follows that because God is this First Mover, God exists.
It seems to me that if all movement must be caused by some being, it follows that even God would have to have a mover in order to move. This supports the idea of an infinite succession of movers. Even though this type of critique is fully justified, most people and cultures have a line of thought that makes us believe that a definite beginning and end exist - an alpha and an omega - I think that it is more sensible to think that movement has always existed, and that there is thus no need for a First Mover.
Another interesting argument Aquinas presents is based on a conception of possibility and necessity. Thomas thought that every possible event occurs at some point in time. As all existing things could also not exist, there was once a time when nothing existed. If such a point in time did exist, then nothing could be created either, as nothingness cannot become anything by itself. It is evident, that at least some things exist in the present moment, which means that even in the time when nothing existed, something had to exist in order to create something out of the nothing. Therefore, God had to exist even when nothing else