THE LEIBNIZIAN COSMOLOGICAL ARGUMENT
This argument is a more airtight form of the normal cosmological argument, developed by Gottfried Leibniz, which appeals to the fact that things need reasons to exists (based on the Principle of Sufficient Reason).
We live in a world of contingent things, that is, everything we see only exists because some reason led to its existance. If your parents didn't exist, you wouldn't exist. If the big bang didn't happen, the planet wouldn't have formed. There is no reason to believe that the universe itself doesn't behave this way; so if the universe has a reason to exist, why does it exist? The answer, I argue, is found in God.
IMPORTANT DEFINITIONS FOR THIS ARGUMENT
Contingent being: A being which does not exist by neccesity of its own nature, and thus relies on external reasons for its existance.
Necessary being: A being whose existance is grounded in the necessity of its own nature; it is impossible for it to not exist, as its nature prevents it from being so.
The Universe: The natural world, time, space, matter, energy, and all natural processes such as quantum fluctuations, which we observe.
THE ARGUMENT
Premise 1: Anything that exists has an explanation of its existence, either in the necessity of its own nature or in an external explanation.
This premise should be clear to anybody seeking truth. It's a fundamental principle of scientific inquiry, and almost any inquiry, to ask why things are the way they are. Whenever we see something, it calls for an explanation. Now, either the reason the thing exists is the necessity of its own nature, which means that its nature necessitates that it must exist, or it does not exist by necessity of its own nature. If it doesn't exist by necessity of its own nature, then it is contingent, either on itself or on something else. However, if something is contingent on itself, that would lead to circular reasoning, as you would be presupposing the existance of itself before you explain its existance, which is absurd. So something that is contingent must be contingent, to a certain degree, on an external reason.
Premise 2: If the universe has an explanation for its existence, then that explanation is grounded in a necessary being (God).
The universe is contingent (see my Causal Argument), so it must have an external explanation. Now, if it requires an external explanation, that explanation could be something that also is contingent. However, this would lead to an infinite regress.
Our goal is to find a reason why the universe exists, and an infinite chain of contingent reasons does not answer this. Assume a friend were to walk up to you and punch you, and when you asked him why he punched you, he said this:
"My reason is that I have another reason."
Then when you asked him what that other reason was, he said that it was that he had another reason for that. And when you pushed further, he said that he had another reason. Ad infinitum.
Even if your friend said to refer to "another reason" infinity times, it transparently is not a reason. An infinite amount of non-reasons put together does not make a reason.
As atheist philosopher David Hume said,
"But you cannot procced after this matter, in infinitum, you must at least terminate in some fact, which is present to your memory or senses; or must allow that your belief is entirely without foundation."
So, the explanation of the universe cannot be a contingent chain, but a necessary being which has its explanation of its existance grounded in its nature.
Premise 3: The universe exists.
I'd imagine this isn't too controversial.
Conclusion: Therefore, a necessary being (God) exists.
OBJECTIONS TO THE FIRST PREMISE
This argument seems to depend on the fact that time is tensed. What about B-theory time, which proposes that the past, present, and future all exist at once, and therefore nothing comes into existance because all past and future states of the universe exist already? Doesn't this premise depend on objects having causes and coming into existance?
When I say that everything that exists needs a reason, I do not suppose that it is a temporal cause that happens before the object in question comes into existance. I am simply stating that things need reasons to exist, nothing more, nothing less. Even a 4-dimensional block universe described by B-theory time still calls for a reason as to why it exists.
OBJECTIONS TO THE SECOND PREMISE
But God can't exist, because an omnipotent being can't make a rock so big that he can't lift it, therefore meaning he is not omnipotent, and therefore making God (an omnipotent being) impossible!
You'll notice that this argument does not prove that an omnipotent being exists. It proves that a necessary being exists. However, that objection still does not rule out the possibility of an omnipotent (all powerful) being. Most philosophers don't crudely define omnipotence to be "the ability to do anything" because that leaves room for paradoxes. Rather, most define omnipotence as the ability to do anything that is possible.
Asking if God can create a rock so big that he can't lift it is like asking if God could create a square circle, or a married bachelor, or the smell of blue. All are logically impossible questions; they're as valid as asking "could God 11 a giraffe?" Why does it matter if God is unable to provide an answer for a question that doesn't make any sense in the first place?
In fact, for an atheist to consider such a question as logical is a critical failure in their entire system of logic; if one illogical statement is held to be true, then anything illogical can be held to be true.
Okay, I get that the cause has to be necessary. But why can't it be a necessary substance? Why must the neccesary cause be conscious?
A necessary substance cannot act on its own- in order for a substance to do anything, it must be caused to do something. But if it is caused to do something, it would either be caused by a necessary substance or a necessary being. If it was caused by a necessary substance, ad infinitum, then it would simply result in an infinite regress, thereby requiring us to terminate in a necessary being, which can make decisions independently.
OBJECTIONS TO THE THIRD PREMISE
Quantum physics and the measurement problem seem to prove that the universe only exists in our mind. Doesn't idealism refute this argument?
Idealism absolutely does not refute this argument. Even if the universe is mental, that does not mean that it doesn't exist, but simply exists in a different state.
OBJECTIONS TO THE CONCLUSION
You may have proven that a God exists, but this doesn't prove that YOUR God exists! Why can't this necessary being be a Spaghetti Monster, or Xenu, or any other idea of God? Why does it have to be YOUR God?
This argument doesn't aspire to prove that "my" God exists, so this is no actual objection. But to answer the question, I believe in a general God because of general theistic arguments like this. To decide what specific God I want to believe it, I examine the evidence for each religion and decide which one seems the most reasonable.
Furthermore, a God like a Flying Spaghetti Monster wouldn't work. Why? To have created time, space, matter, energy, at the complexity that we observe, the God must be timeless, spaceless, immaterial, enormously powerful, and personal. The Spaghetti Monster must not be made of matter, must not occupy space, must not be bound to time, have the power to create everything, and have intelligence. But at this point, the Spaghetti Monster sounds less like a monster made of spaghetti and more like God.