Every effect demands a cause. There cannot be an infinite chain of causes and effects, so an uncaused first-cause, a prime mover, is required. God is that first cause.
This is the crux of the First Cause (Cosmological) Argument; a hypothesis that is at once the most exercised and weakest explanation for the existence of a god. Turning a blind eye to the assertion that “there cannot be an infinite chain of causes and effects” (why not?) if cause and effect is required there can be no uncaused cause. If there can be an uncaused cause (ergo a first mover god), cause and effect isn’t required. To allow an exemption is to say cause and effect is not in any way, shape or form compulsory. The argument however says, “cause and effect is required but because I can’t count to infinity a first cause is also required, and since there cannot be an uncaused cause, yet one is required, ‘God’ exists.” In case you missed it, this is the argumentative equivalent of dowsing yourself in high octane fuel then (with a perfunctory smile) sparking a match. It arbitrarily defines ‘God’ as something that is exempt from the cause requirement, yet the only reason it exists (based on this argument) is for the purpose of satisfying that requirement. The hypothesis circles back in on itself and to anyone with half a brain vanishes in a puff of illogical vapour.
*Parts of this post taken from Existential Atheism.