It was William Lane Craig, I believe, whom I first saw formulate the syllogism in this way:
1. If objective moral values exist, then God exists.
2. Objective moral values exists (i.e., evil is real, not illusory).
3. Therefore, God exists.
The fact that atheists must presuppose the very thing they intend to deny fits well with the colorful illustrations that Cornelius Van Til used to use in order to demonstrate the fundamental inconsistencies of all non-Christian worldviews. Van Til argued that non-Christians were operating on "borrowed capital"--using the Christian worldview in order to destroy it. They couldn't help themselves. They need to use fundamentally theistic categories--like laws of science, morality, and logic--as tools to defeat theism, and yet they cannot account for them on their worldview.
Van Til compared this to a little girl sitting in her father's lap, slapping him in the face. She must be supported by him in order to rebel against him. Another time he spoke of the futility of non-Christian thought as being like a man made out of water using a ladder made out of water in order to climb up out of water!
Stand to Reason's Greg Koukl would not, I believe, want to be lumped in with Van Til, but I think that his article on "Evil as Evidence for God" is a fine illustration of the way in which non-Christians presuppose what they intend to deny. Here's how Greg begins:
Read the whole thing.
The presence of evil in the world is considered by some to be solid evidence against the existence of God. I think it proves just the opposite. The entire objection hinges on the observation that true evil exists "out there" as an objective feature of the world. Therein lies the problem for the atheist.
To say something is evil is to make a moral judgment, and moral judgments make no sense outside of the context of a moral standard. Evil as a value judgment marks a departure from that standard of morality. If there is no standard, there is no departure.
Evil can't be real if morals are relative. Evil is real, though. That's why people object to it. Therefore, objective moral standards must exist as well. This discovery invites certain questions. Where do morals come from and why do they seem to apply only to human beings? Are they the product of chance? What world view makes sense out of morality?
We can answer these questions by simply reflecting on the nature of a moral rule. By making observations about the effect--morality--we can then determine its characteristics and then ask what cause is adequate to produce it.