I understand that many people believe that, but I don't take it that there is broad consensus-- or any consensus-- that the Big Bang represents the beginning of anything save our the beginning of what we can reasonably infer and observe. I think most self-respecting astrophysicists would respond to the question "Was there anything prior to the Big Bang?" with an "I don't know".

Regardless, P1 is still required to make Craig's KCA work, because P2 is compatible with the universe just happening.

Editing to account for your later comments:

>There is nothing that comes into existence without a cause. From your innermost creative thoughts to a spot of mold on bread, to a new leaf on a tree, to the seemingly random emergence of a rain storm - all have a beginning and all have a cause.

That isn't quite the same scenario. The sort of causation we observe (which is actually compatible with no causation, a la Hume) is more like shuffling around of existing matter and does not translate to direct evidence for or against the hypothesis that something can come into existence uncaused.