A naturalist believes the Universe is expanding because of a big explosion from a state of nothingness. A theist believes the Universe is expanding because a Supreme Being spoke it into existence out of a state of nothingness, much closer to the Earth, then moved it outward leaving a trail of light coming back to us at the speed of light. Either possibility would yield the same results, as far as what we observe.
Both beliefs are based on philosophical worldview interpretations. All we know is that the Universe exists and appears to be expanding.
The Law of Cause and Effect would indicate that everything that exists in our whole universe has a cause. But was it a natural cause or a supernatural one? Will which ever explanation you and I have chosen have any effect on the reality of what actually happened?
Until we each learn to separate our search for truth from our chosen philosophical worldview; our search for absolute truth will always be hindered.