Defining God and applying the defined term to naturalism...?

According to the naturalist, the universe is expanding and thus inherently began recently. Because all causes require an effect by nature, something must have produced the observable cosmos. By nature, this "creator", rather he be intelligent and personal or dimwitted and impersonal, must be transcending and eternal. Is a transcending and eternal creator considered a God? If so, why do naturalists such as atheists claim to believe that a "God" is not necessary in the creation of the observable cosmos?