Why do you not believe in God?

[jerNYC]

So what KCA is attempting to do, is tie the first two premises with beginning to exist. The issue is, when you introduce the "begin to exist" condition, you automatically imply that some things DO NOT begin to exist. Hence the question begging, because you are attempting to prove that some cause, which does not begin to exist, is THE CAUSE, or god, or whatever else your argument is trying to prove.

I agree with both you and CliveStaples. The original post was poorly phrased. Had it been phrased like the argument below, it would have been a pretty simple argument and I would agree with CliveStaples' formal proof.

1. All momentum has an initial action
2. The universe has momentum
3. Therefore the universe had an initial action which set it into momentum

For this argument to be valid, it has to be true that the universe has momentum. I assumed when the original post stated (Big Bang) that he meant the universe has momentum, since it's expanding from a single point in space. The argument is valid.

But the wording in the original post could be interpreted as begging the question. It all depends on how you define terms like "cause" and "begin to exist."