Sometimes, when arguing that the universe requires a self-existent first cause, I'm told by Christians that I ought not argue the way Aristotle did because his god, the Unmoved Mover, was an idol and not the true God.
Here are some words from R.C. Sproul responding to this charge (transcribed from Sproul's lecture, "God of the Bible vs. God of Philosophy"):
What I hear from people objecting to our labors to construct the self-existent, eternal being is that that only gets us to the First Cause, and to the God of the philosophers, and it doesn't get us to the God of the Bible, therefore the approach is false, and what we come out with is false because it is not the complete picture. It's only a partial picture of the true God.
Now, the question is this: Do we have to have a comprehensive knowledge of God in order to have true knowledge of God?...If indeed we would have to have a total, comprehensive picture of God in order to have a true understanding of God, then this would only mean…that we have no true knowledge of God. Because we certainly don't have a comprehensive knowledge of God. In other words, what I'm saying is, even if our knowledge of God is partial, that does not mean that it's untrue. It is true as far as it goes, and even though we grant that what we've achieved so far in our reasoning process is only to get to a self-existent, eternal being, that that certainly is part of what the Bible reveals to us about the character of God. Because whatever else the Bible reveals about the nature of God, the Bible certainly teaches that He is eternal, that He is self-existent, and He is the One who is the Creator of all things.
Now at that point, Aristotle says "yea, and amen." Is Aristotle wrong? No. And the fact that a pagan philosopher agrees that there has to be a self-existent, eternal being does not vitiate the truth of the Christian claim. In fact, it agrees with it. And we're saying, yes, we agree with Aristotle in the sense that there has to be a first cause, and that that first cause has to be self-existent, and that that first cause has to be pure actuality, and that first cause has to be pure being, and that that first cause has to be eternal. We say, thank you very much, Aristotle--you agree with us, we agree with you, on this cardinal point, which is only a partial point of our knowledge of God. But it is a crucial portion of our knowledge of God because, ladies and gentlemen, it is precisely this aspect of the Christian understanding of God that is constantly under attack by atheistic systems of thought…And so I think there's great value in establishing that not only faith but reason, as well, demonstrates the logical necessity of having a self-existent, eternal being.