As far as Kalam goes, I think short of obtaining a much clearer definition of “existence” we can easily reject the theist’s entire argument immediately by not granting the validity of premise #2 and by noting the definitional incoherence between the two premises.
1. Everything that has a beginning of its existence has a cause of its existence.
2. The universe has a beginning of its existence.
It seems as if the first premise is referencing “existence” as being the result of observable cause and effect relationships that we see in our known universe. E.g. a particular human being “begins to exist” in the sense that, thanks to its parents, a unique configuration of molecules has become arranged in such a way that they give rise to a complex sentient entity. At no time did any constituent part of such a new human “come into existence”. All the atoms pre-existed and were simply rearranged and assembled.
Conversely, “existence” in the second premise is almost always used by theists in the context of the universe becoming “something” from literally “nothing” – as in the philosopher’s conception of absolute non existence. It’s a state where, by their own definition, no matter whatsoever pre-exists.
The simple fact is that we have no evidence of anything ever coming into “existence” in the sense of the former. From what I understand of current cosmological theory, this includes the known universe itself as well. As William Lane Craig likes to point out, cosmologists theorize that time and space were created at the Big Bang but he conspicuously leaves out the fact that no physicist ever makes the claim that mass was created as well. As far as I know, the definition of the Big Bang singularity includes a description of a point of “infinite mass”. Infinite mass sounds a lot like “something” to me. As far as we know, the universe did not come from the philosopher’s version of “nothing”.
On important aspect to note is that the philosopher’s concept of “nothing” is contrasted and confused (and in many cases, probably misrepresented) with the physicist’s current working definition (see Lawrence Krauss et al) which references the quantum vacuum of empty space which they’ve found not to be quite so empty.