1.Every finite and contingent being has a cause.
2.A causal loop cannot exist.
3.A causal chain cannot be of infinite length.
Therefore, a First Cause (or something that is not an effect) must exist.
This is by far one of the more difficult arguments to counter. I guess the best response would be that we currently don't have the answer, but that doesn't mean science won't figure it out, and just because it's currently unknown doesn't mean God did it. There was a time when we didn't understand what caused lightening, so we attributed it to Thor, Jupiter, and many other gods. Lawrence Krauss has been publicizing the idea of "A Universe From Nothing." Basically, quantum fluctuations which pop in and out of existence account for most of the mass and energy, and yes, scientifically, something can come from nothing.You are the 50th visitor