In one sense first member of a causal series is a part of the series, but in another sense it isn’t. If ABCD causes something, then A is obviously 1/4 of all the causes you have, but we don’t think about it that way. We don’t say that George Bush played a part in the Iraq War, or even a crucial part in it – it was just his war. Truman wasn’t a part of the system that dropped the bomb – the system was brought int existence by his choice. This is true in every genus of causes. Winning isn’t one part of an athlete’s goals, even if one can isolate other goals than this in the game or in training. A fire hydrant is red and a light wave in the right spectrum is red, but the “is” is not said in the same way. The two things “are red” but not in a way that the one is a part of the whole.
The simplest account of “God is not a being” the debates over univocity and analogy might be approached in light of this: taken as an abstract series God and creation are both beings, but when we consider causal series things change since the first member of a causal series, as first, isn’t considered as a part or as taking part in the action of the whole.