For something as big as the existence of God, really nobody definitively 'knows'. Nobody should be pompous enough to say they can know for certain whether God does or doesn't exist. Even Richard Dawkins says he's a 6.9/7 on his Theist/Atheist scale.
Belief in God relies on faith, which is suspension of the evidence. One dictionary defines faith as 'based on spiritual apprehension rather than proof.' Most christians rely on personal experience, anecdotal stories, or spiritual revelation as 'evidence' that God exists. However, on a scientific level, the idea of a God cannot be tested. This is a section from a very good article called 'Weighed and Found Wanting'. You can probably find the .pdf on here, but anyway:
Clearly, different evaluative criteria are required to determine the merit of a theory. Four features must be assessed: explanatory scope, simplicity, plausibility and testability. . . In light of these criteria, supernatural explanations are ruled out of scientic investigation. This is not a prejudice against supernatural causes, but an inherent limitation of science. Why? Because supernatural causation, or claiming 'God did it' as a scientific conclusion, fails by nearly every measure of what constitutes a good theory.
It is reasonable to propose that God has maximal explanatory scope, but the nature of the explanation is vague. When God causes an animal to exist, is it the same process as when he creates an angel, or a planet? How does this process work? While it is easy to answer any mystery by invoking an omnipotent power, and thereby yield a huge explanatory scope, upon scrutiny these explanations quickly fracture into many ill defined sub-explanations.
Simultaneously, these smaller explanations reduce the simplicity of supernatural causation. Additionally, even if a minimal Creator is the only thing being proposed, this is not the type of God the majority of people believe in. Most religions incorporate an active God who intervenes in history, reacts to human actions, and makes proclamations about morality and the afterlife. All of these extra details lead to a very complex theory.
What about plausibility? Supernatural beings are, by definition, not natural and do not obey natural laws. Hence, all current natural knowledge is useless for evaluating the actions of a nonnatural actor. Because the supernatural is not constrained by the natural, it is impossible to determine plausibility. Does God creating life fit in with what supernatural beings are known to do? No one can say, because there is no background knowledge about the supernatural.
The final, and most important element of science is testability. Scientic claims must be testable, but God is not testable. Individual claims about a god may be tested, but it is always possible to invent a special case that sidesteps the evidence. For example, studies have shown that intercessory prayer does not help hospitalized people heal faster. [19] However, believers can simply assert that God does not heal people when they are part of a scientific study, thereby avoiding all objective empirical methods. Similar arguments can be used to avoid any piece of evidence. One can also continually move the divine back one step to maintain supernatural causation. Perhaps someone believes that Zeus causes lightning. A scientist could explain that lightning is actually static electricity resulting from the friction of particles of precipitation in a cloud. The believer can simply incorporate this information by claiming that static electricity is important, but that Zeus is the real "cause" of the lightning. It is impossible to scientifically validate or invalidate this theory because it is beyond the methods of science.
At the Deist level, Gods attributes are unknown. However, the supposed qualities of the Theistic God, given to us through 'inspired' books like The Bible and Koran, CAN be tested. It's in this area that the God idea falls flat, at least in my opinion.
