The argument goes like this:
An "actual infinite" cannot exist. That is, anything that exists must have a beginning and an end.
Therefore, the universe must have had a beginning, and therefore a cause.
(Here's the first giant leap of faith) "God" is this Cause.
Then you can identify this "God" with the God of the Judeo-Christian tradition, with absolutely no grounds for doing so, other than the fact that you were raised in that mythology.
Of course, the obvious objection to this whole MO is that we begin with the premise that an "actual infinite" cannot exist - so what about God? If God exists, must He not likewise have a beginning and an end, and therefore a cause? So what caused God? But the excuse is that we've defined "God" as the exception to the rule.
Well, good for God; but this type of nonsense fails to satisfy me.