Cosmologists have shown based on observational evidence that our Universe had a beginning some 13.8 billion years ago which is traditionally called the Big Bang event. Logic, or at least intuition dictates that this Big Bang event had a cause. There must have been some reason why the Universe came into existence. Theists however amend this logic to intuitively say, actually state, actually conclude that there was a reason for this act of creation. God did it. Alas, that conclusion doesn't arise of necessity from the premises.
The Kalam Cosmological Argument as oft stated by theists, most notably William Lane Craig, is as follows.
1) Premise: Whatever begins to exist has a cause.
2) Premise: The Universe began to exist.
3) Conclusion: Therefore, the Universe has a cause.
If the argument stopped there, well all's well that ends well. Relatively few people would have problems with the cosmological argument as given above. But, and there is always a “but” to have to consider, theists like William Lane Craig immediately leap to one further conclusion.
4) Conclusion: Therefore the cause behind the existence of the Universe was God* because the entity behind the creation of the Universe had to have been itself uncaused, beginning-less, changeless, eternal, timeless, space-less, an immaterial all powerful being who is a personal agent, endowed with freedom of the will. [To be quite honest, this is yet another pure leap into a philosophical God-of-the-gaps conclusion. If the cause is unknown, if there is a gap in our knowledge needing to be filled, the unknown must be God.]
Allow me to amend the above slightly.
1) Premise: Whatever begins to exist has a cause.
1A) Firstly this is just an appeal to intuition and intuition isn't always a pure pathway to truth (i.e. - intuition states that the Sun goes around the Earth). There may indeed always be a cause for anything and everything that has or ever will come into existence, including whatever came into existence at the Big Bang event (the postulated beginning of our Universe), but that cause isn't always evident. Some quantum physicists would in fact claim that there are uncaused things (i.e. - radioactivity).
1B) Whatever cause in itself that has come into existence has, IMHO, thus resulted from a previous cause, which had a previous cause which had a previous cause and that chain can be extended as far back as you wish. Stated another way, there is no such thing as a First Cause.
1C) Whatever thing that came into existence came into existence from a previous thing(s) which existed and which in turn came into existence from a previous thing(s) which in turn came into existence from yet a previous thing and so on as far back as you wish to go. Stated another way, you can only bring something into existence from a previous something. You cannot bring a material something into existence from pure nothingness or from anything immaterial.
2) Premise: The Universe began to exist.
2A) I need note here that the "Universe" is defined as the sum total of all the bits and pieces that collectively make up the, or our, "Universe". The "Universe" is just the label we give to all of those bits and pieces (particles, atoms, molecules, dust, rocks, planets, stars, etc.) that came into existence in-the-beginning or later emerged into existence out of simpler states (i.e. – molecules from atoms).
2B) The assumption here is that our Universe is the be-all-and-end-all of the Cosmos**. While that may be the case, it's not necessarily so. Just because you came into existence doesn't mean that others don't also exist. Our Universe could be one of many. There could be parallel universes or even a postulated Multiverse or Megaverse - maybe.
3) Conclusion: Therefore, the Universe has a cause.
3A) The effect (resulting from the cause) of the Universe coming into existence or coming into being is called the Big Bang event, so the cause of the Universe (i.e. - the cause of the Big Bang event) was something prior to the Big Bang event. If the Universe had a cause then that cause was obviously pre-Universe or before the Big Bang event.
3B) That's where the cosmological buck has to stop since we can't observe or measure anything prior to the Big Bang event.
3C) In context all we can say is that our Universe came into existence at the moment of the Big Bang event and that the Big Bang event had a cause. That says nothing about the larger context as suggested in 2B. It could be that our Universe popped into existence from within a larger Cosmos just like a baby pops out of the womb at birth.
4) Conclusion: Therefore the cause behind the existence of the Universe was God.
4A) Nearly all theists state that the cause of the Universe was due to an omnipresent (all-present), omniscient (all-knowing), omnipotent (all-powerful), all-loving, perfectly moral, and perfectly benevolent Almighty Being (i.e. - God). However these traits along with an entity who is itself uncaused, beginning-less, changeless, eternal, timeless, and space-less; an immaterial all powerful being who is a personal agent, endowed with freedom of the will, aren't verified; aren't all mutually inclusive and logical, with many an inherent philosophical inconsistency as well as many being actually contradicted by Biblical chapter-and-verse passages (i.e. - God is hardly all-loving).
4B) But a supernatural deity with some or all of these traits is also a total fallacy even if for no other reason than that the Cosmos has to be eternal (temporally infinite) since as I noted above there can be no First Cause and because you can't, and not even God can, create something material from the immaterial. It's a logical contradiction to postulate the creation / existence of an absolute something from an absolute state of pure nothingness and even God has to conform to logic (i.e. - God can't create a spherical cube). If you can't create something from nothing then something has always existed. If the Cosmos is infinite or endlessly cyclic, an infinitely repeating causal loop where A causes B and B in turn causes A, then what need for a God? If therefore, as theists want, that the Cosmos is finite since infinities aren't possible (i.e. - they tend to throw spanners into theistic philosophies - see 4D), then God too is temporally finite, therefore had a beginning and therefore had a cause. That of course contradicts the concept of an eternal deity and raises the obvious question, what caused God? If God is eternal then God created the Cosmos and our Universe an infinite time ago which is clearly not the case.
4C) Since science can't explain or actually identify the "cause" that caused the existence of our Universe, on the grounds that the cause preceded the Big Bang event and thus this cause can't be observed or measured, theists step into the gap and conclude that God is that cause. This God-of-the-gaps conclusion is also a fallacy since there are numerous other alternatives. The cause of the Universe could have been the Flying Spaghetti Monster or any deity or deities from any of the world's hundreds of creation mythologies. Maybe it was just a natural Big Crunch (a contracting universe) turning inside out at crunch time into a Big Bang; maybe an unknown and perhaps unknowable other natural cause we haven’t imagined yet; perhaps a quantum fluctuation; even perhaps (and this is my bias) a mortal, fallible, flesh-and-blood computer / software programmer fills the gap. God is only one hypothesis of many.
4D) Theists, even some cosmologists mistakenly say that there can't be an infinite Cosmos due to entropy (the state of useable energy available). An infinite Cosmos would have attained a state of maximum entropy an infinite time ago but that is not what we observe. I contend that at the moment of the Big Bang the clock was reset to time equals zero; the Universe was restored to original factory settings (including a state of minimum entropy). Consider this analogy. You only started ageing, started running down, and started increasing your entropy, at your conception. That's when your clock started. That state of conception was your original factory condition. What came before was irrelevant since as far as you are concerned, there was no before (although clearly there was). You had a cause therefore there was a state that existed before you. That cause was your parents and their state of entropy is an irrelevance as far as you (their child) is concerned at conception.
So the Big Bang is analogous to your conception. What caused the Big Bang is analogous to your parents. The state of entropy before the Big Bang and before your conception is irrelevant to our Universe and your conception.
* Your own personal version of God of course is The God of choice - of course.
** The Cosmos being all that ever was, is or ever will be.