Many (but not all) atheistic detractors of the First Way (the argument from change) often misunderstand the purpose of the argument. Why, they ask, does the Unmoved Mover have to possess the attributes of God? This is a strange question, to say the least. After all, Thomas Aquinas spends very little time on the Five Ways, and much more time offering demonstrations that the Unmoved Mover/First Cause is God. It's almost as if Richard Dawkins, an even the philosopher Michael Martin, haven't bothered to read the rest of Thomas' writings that bridge the gap between Unmoved Mover/First Cause and God. At least Martin doesn't succumb to the "what caused God?" objection, which is laughable to virtually all atheistic philosophers.
With that said, the argument from change has two parts: a) demonstrating the existence of an Unmoved Mover; and b) demonstrating that the Unmoved Mover is God. In this post I will only focus on the first major contention. The argument may be summarized as follows:
1. Changing things exist. (Premise)
2. Changing things exhibits actuality and potentiality. (Premise)
3. No potentiality can actualize itself. (Premise)
4. Either an Unmoved Mover exists, or there is an infinite regress of potentialities being actualized, or there is a circularity of potentialities being actualized. (From 1 - 3)
5. There cannot be an infinite regress or a circularity of potentialities being actualized. (Premise)
6. Therefore, an Unmoved Mover exists. (From 4 and 5)
Virtually nobody doubts premise (1). To doubt it and maintain that change is illusory not only affirms the existence of Pure Actuality (which the atheist does not want to do), but to come to the conclusion that all change is illusory itself constitutes a change.
Premises (2) and (3) are easily defensible. In support of premise (2), an acorn is merely an acorn in actuality, but is an oak tree in potentiality. Premise (3) can be defended by pointing out that if a potentiality were to actualize itself, then it would both exist in potentiality and actuality simultaneously, which is contradictory. Moreover, we have many examples that confirm this premise. An acorn doesn't become an oak tree all by itself. Rather, it requires sustaining causes of its change, such as water, sunlight and soil. If any of these sustaining cause were removed, then the acorn's development would cease to continue.
(4) logically follows from premises (1) through (3), so what about premise (5)? First, why can there not be an infinite regress of potentialities being actualized? Thomas Aquinas offers three distinct reasons, but I'll only appeal to one. Even if the past is infinite (and empirical evidence makes such a scenario virtually impossible), the past is still composed of finite periods of time. At each finite period of time, the regress of sustaining causes of change (not originating causes) begins anew. This means that during a finite period of time, there cannot be an infinite regress of sustaining causes of change. Why not? The reason is simple. Given that the regress begins anew, it is impossible to arrive at infinity. No matter how many numbers are counted, there will always and indefinitely be another number to count before arriving at infinity. Hence, it is impossible for the regress of sustaining causes of change to be infinite.
Now, what about a circularity of causes of change? This is even easier to eliminate as a possibility. We have already established that no potentiality can actualize itself. With a circularity, say, A causes B to change and B causes A to change at the same time and in the same sense, A and B would have to exist in both potentiality and actuality simultaneously. Since this is impossible, the only conclusion we can make is that an Unmoved Mover exists.
Now, objections to an Unmoved Mover I've found to be quite weak. One common objection is that in order for A to change B, A must also change. However, imagine a beautiful painting. While gazing upon the painting, a man is drawn to it (a change) because of its beauty. Of course, a painting, being a material thing, is changing in terms of time and molecular movement. However, the Unmoved Mover is immaterial, as is demonstrated in the second major contention. Therefore, the attempt to point to a disanalogy fails as an objection.
The "what changes the Unmoved Mover?" objection is perhaps the worst objection I'm aware of. The causal premise states that no potentiality can actualize itself, and yet, the Unmoved Mover does not exhibit any potentiality. Rather, the Unmoved Mover is Pure Actuality, as the second major contention states.
I look forward to expanding on these arguments, including the second major contention, in my Master's thesis for an M.A. in Philosophy. All I can say for now is that the objections to the First Way I've found to be extremely weak, sometimes based on misinterpretations, and other times simply due to not doing enough research.