1. Whatever begins to exist has a cause. (Premise)
2. The universe began to exist. (Premise)
3. Therefore, the universe has a cause. (From 1 and 2)
If this argument is sound, then the cause of the universe (the sum total of all physical space, time, matter and energy) would have to be timeless, changeless, immaterial and very powerful. I won't comment on premise (1), except to note that it relies on the ex nihilo principle: out of nothing comes nothing.
I'm more interested in premise (2). William Lane Craig offers two philosophical arguments and two scientific arguments in support of this premise. I only want to focus on one of his philosophical arguments.
2a. An actual infinite cannot exist in the real world. (Premise)
2b. A universe without a beginning includes an actual infinite in the real world. (Premise)
2c. Therefore, the universe's past must be finite. (From 2a and 2b)
I'll skip (2c), since I assume it's not the controversial premise of the argument. Why think an actual infinite cannot exist in the real world? Here's what I've gathered based on my on-and-off study of the KCA:
2i. In set theory, subtraction and division are prohibited when applied to infinite sets. (Premise)
2ii. Nothing in the real world would prevent subtraction or division when applied to any set. (Premise)
2iii. Therefore, there cannot be an actual infinite in the real world. (From 2i and 2ii)
Keep in mind that set theory is logically consistent. If a mathematician wants to prohibit certain functions in a mathematical theory, that's fine. The question is whether or not such a theory can be applicable in the real world.
I've struggled with this argument, but I do see a lot of intuitive support for it. Nevertheless, I'll ultimately leave this issue to those who are experts on the KCA.