One reason why I'm a monotheist: things change. Shocking, right? Why would change indicate the truth of monotheism? Moreover, why would I maintain that evolution is evidence *for* God's existence (evolution is change by definition)? If you're interested, please bear with me; this will be a fairly lengthy post. I'm also writing my Master's thesis on this argument (St. Thomas Aquinas' First Way).
1. Changing things exist. (Premise)
2. Everything that changes is composed of actuality and potentiality. (Premise)
3. No potentiality can actualize itself. (Premise)
4. Either an Unmoved Mover exists, there is an infinite regress of causes of change, or there is a circularity of causes of change. (Implied by 1, 2, and 3)
5. There cannot be an infinite regress of causes of change nor a circularity of causes of change. (Premise)
6. Therefore, an Unmoved Mover exists. (From 4 and 5)
By "Unmoved Mover" Thomas means Unchanged Changer, since his use of "motion" refers not to just movement from one location to another, but to change in general. "Unmoved Mover" just sounds more aesthetically pleasing, in my opinion. So, let's consider each premise first.
Premise (1) is evident to the senses. Things change. In fact, if change were an illusion, then the realization that change is an illusion would itself be a change, which is contradictory.
Premise (2) is supported by the fact that there are things that exist that could change into something else. An acorn, for instance, could grow into an oak tree.
Premise (3) should also be accepted without reservation. This is because if a potentiality were to actualize itself, it would have to exist and not exist simultaneously, which is another contradiction.
Premise (4) is implied by the first three premises. There is no additional alternative.
Now, let's pay close attention to premise (5). First, why can there not be an infinite regress of causes of change? The reason is simple. Even assuming the past is infinite (which is dubious at best), it is still composed of finite parts. At each finite period of time, the regress of sustaining causes of change is renewed. It is impossible to count to infinity when one begins counting. This is because it is always and indefinitely possible to count another number before arriving at infinity. Hence, there cannot be an infinite regress of sustaining causes of change. So, what about a circularity of causes? This too is impossible, since both causes would have to exist in potentiality, which means they would have to actualize themselves. We have already seen this is impossible.
As a result, there must exist an Unmoved Mover. Now, what are some attributes of the Unmoved Mover? Since the Unmoved Mover is immutable (or else it would have to be actualized, which is another contradiction), it must be Pure Actuality. What about Pure Actuality is God-like? Here are a few attributes we can deduce. First, Pure Actuality must be eternal and indestructible. This is because an immutable entity cannot come into being or cease to exist, since that would entail a change. Next, the Unmoved Mover must be immaterial, since physical entities have the potential to change.
What else? The Unmoved Mover must be unique. If there were more than one Pure Actuality, then there would be distinctions between the two. However, to be distinct from actuality is to be non-actuality, in which case the latter does not exist. Also, the Unmoved Mover must be omnipotent, omniscient, and perfectly good. Why? Because a lack of any of these attributes would entail potentiality (it could become more powerful, etc.), and the Unmoved Mover is Pure Actuality.
What do we have then? We have demonstrated the existence of an Unmoved Mover who is immutable, purely actual, eternal, indestructible, immaterial, unique, omnipotent, omniscient, and perfectly good. In the words of St. Thomas Aquinas, "this everyone understands to be God."