The Kalam cosmological argument includes this bit of reasoning:

1. The universe began to exist.
2. If the universe began to exist, then something (other than the universe) caused the universe to begin to exist.
Therefore,
3. Something (other than the universe) caused the universe to begin to exist.

This reasoning seems fine to me. The universe did not come from nothing; it came from something. The disagreement between atheists and theists is (or should be) over the nature of the something from which the universe came.

For atheists, the X that caused the universe to come into being was something that was unthinking or impersonal, whereas for theists the X that caused the universe to come into being was something personal or mental. The universe is the product of a person or mind, according to theism, and it is not the product of a person or mind, according to atheism.

One qualification needs to be made here. From an atheistic or naturalist point of view, the universe could be the product of a person or mind so long as that person or mind was itself produced by purely physical or impersonal forces, or so long as the ultimate source of a causal chain of persons (that produced the universe) was some unthinking and impersonal thing.

A similar qualificaiton needs to be made about theism. From a theistic viewpoint, the universe could be the product of an unthinking or impersonal thing so long as that unthinking or impersonal thing was brought about by a mind or person, or so long as the ultimate source of a causal chain of unthinking or impersonal things (that produced the universe) was some mind or person.

Alas, it appears that the difference between atheism and theism concerning the origin of the universe, is not as clear and straightforward as one might wish. Atheism is compatible with the universe being the product of a person or mind, and theism is compatible with the universe being the product of an unthinking or impersonal object or force. The disagreement is really about the ultimate source of the universe.

So, even if we can definitively establish that the cause of the existence of the universe was a mind or person, that would not be sufficient to rule out atheism, for that person or mind might in turn be the product of unthinking and impersonal forces. Similarly, even if we can definitively establish that the cause of the existence of the universe was some unthinking or impersonal force of nature, that would not be sufficient to rule out theism, for that unthinking or impersonal force of nature might in turn be the product of a mind or person.