The God Delusion's central claim is stated early and repeated throughout: a god's existence is a scientific hypothesis, exactly like any other claim about how the universe works, and should be judged by the same evidence and the same standards of proof, not exempted into a separate category called faith. Dawkins treats the exemption itself, the idea that religious claims deserve special protection from ordinary scrutiny, as the more interesting target than any specific theology.
That framing drew sustained criticism as much as praise. Philosophers and theologians, including some who shared Dawkins's conclusions, argued that the book engaged the weakest available versions of religious argument rather than the strongest, and that a scientist writing outside his own field on philosophy of religion was not always equipped to represent that field fairly. Even sympathetic reviewers often described the book as a polemic rather than a systematic treatment, useful as a document of a cultural moment rather than as a settled word on the arguments it addresses.
That double reception, commercially dominant and academically contested at once, is itself part of what makes the book a defining document of New Atheism as a movement rather than simply an argument. It reached more readers than any rigorous philosophical treatment of the same questions likely could have, and the arguments about how fairly it made its case became, in their own right, part of the ongoing public conversation the movement is remembered for starting.