The Big Bang theory posits a temporal beginning to the universe, and doesn't explain why the Big Bang happened, or why the universe exists at all. Why is there a universe, as opposed to none? Couldn't it just as easily have been the case that nothing existed? Previous theories, like the Steady State or the Oscillating Universe theory, claimed that the universe has always existed. But these theories have been rejected. With the Big Bang, this is not the case. There was a beginning. So, even if you reject the Bible from the fourth word (In the beginning God), you have to say that the first three words are OK and in accordance with science.
If something begins to exist, and nothing caused it to exist, isn't that a strange thing to say? If we are eating lunch, and a bunny rabbit begins to exist and munches on your salad, would it make any sense to say "Oh, that rabbit just popped into existence out of nothing. It didn't have a cause."