Does God exist? There are many different arguments apologists use to make the case for God's existence, such as why the universe exists at all, the fine tuning of the universe, the existence of moral values and duties, the existence of reason, and so on.
Each of these pieces of evidence for God has merit. However, assuming any one of them is a slam-dunk is a bit foolish. The real power in the arguments comes when we look at all of them together. This is known as making a cumulative case for God's existence. Taken together, the arguments form a very strong foundation, making the belief that God exists much more reasonable than its negation.
I've written previously on the strength of a cumulative case for God's existence. However, an interesting objection to the cumulative case argument was voiced by an atheist named Tyler on the podcast of the UK radio show Unbelievable? hosted by Justin Brierley. Around the 1:09:10 mark, Tyler asked:
What does it mean to make a cumulative case for God? It seems to me the phrase is used to get around one of the oldest debates in recorded history: How many gods are there? I guess you wouldn't have to call them gods; you could just call them supernatural beings. Does the phrase "a cumulative case for God" really eliminate the possibility that the universe was created by one God and the morals by another? Couldn't all of the arguments for God point to a pantheon of supernatural beings that may not ever exist anymore?
Who God is versus arguments for his existenceTyler's question is an important one, not only because it shows a misunderstanding of what the cumulative case is, but also a misunderstanding for the arguments referenced therein. Argument for God's existence, like those listed above, are focused on the idea of necessity. For instance, the Kalam cosmological argument argues for why there is something rather than nothing at all. Such an argument is not limited to only the natural world. If supernatural beings exist, one must account for their existence, too. Those beings are either contingent, meaning they rely upon something else for their existence, or they are necessary--they have always existed, that is they are eternal. Christianity holds that God is an always-existing being that anchors all other existences.
To posit only other supernatural entities that may not even exist anymore runs into a host of problems. First, one must ask "Where do such beings come from? Why do they exist?" I imagine one retort would be "They are all eternally existent!" but this won't do the job. If these beings no longer exist now, it proves they are not eternal.
"Fine," one might say, "They continue to exist as well right now." But we still have some problems. First, there cannot be more than one eternal, all-powerful being. Think about this for a moment. If there are even two beings who claim to have all power, the one thing each absolutely couldn't have is power over the other. If they are equal in power, it means their power is limited by the simple fact the other being exists. So they couldn't be all powerful. Two all-powerful beings is a contradiction in terms. And if they are not all powerful, they are in some way contingent for their power is mitigated. One of the two simply doesn't need to exist since the other can do his job for him.
The moral argument plays out the same way. How can one being be creator and another be the foundation of morals? Does this mean the creator-being is obligated to follow the dictates of the morals-establishing being? Along this line of reasoning, one runs smack-dab into the Euthyphro dilemma Plato spelled out. It again makes these beings contingent, reliant upon something or someone outside of themselves.
Why only monotheism is logically coherentThe Christian who offers a cumulative case for God is doing so in part to explain the existence of contingent things. To suppose multiple supernatural beings then forces the question about their existence, given they are contingent themselves. One must either hold to a contradiction or stumble into an infinite regress, wither of which is a reasonable position to take. Only a single necessary being works consistently given all the evidence presented.
One reason understanding the difference between necessity and contingency is so important is it helps the truth seeker save a vast amount of time exploring different religious faith claims. It shows any faith that posits multiple gods as an explanation for the origin of the cosmos is probably incorrect and monotheistic faiths should be investigated first.