If you've been around apologists or philosophically-minded theists long enough, you've run into someone advocating an ontological argument (OA for short) or some modal argument for God's existence (MAG). Maybe it's a modal cosmological argument, an argument relating to the principle of sufficient reason (PSR), a modal epistemic argument, etc. I, along with many other atheists and some theists, tire of these arguments. MAG proponents often engage in special pleading, ad hoc re-definitions of terms, or simply lack a basic understanding of the concepts involved in their own argument. Hovwever, remember that most MAG advocates are genuine, well-meaning people who either: 1) missed out on some crucial philosophical information [happens to the best of us], or 2) were misled by the most deceptive philosopher I've ever seen: William Lane Craig. Yet as most atheists soon realize, you can explain a MAG's fallacies, equivocations, etc. and proponents still support the argument. So I am writing this brief guide to help combat this problem. Most of my advice will come from my experience with the PSR and the OA (see my exchange with Mentat1231 and my post "Ontological Arguments: A Critique"). However, this advice should aid atheists and theists in combating other MAGs as well. Here we go!
First, let the proponent lay out their MAG. Then ask them to clearly define their terms. YOU HAVE NO IDEA HOW IMPORTANT THIS STEP IS. For instance, presuppositionalists, the class of apologists who most frequently run OAs, are famous for equivocating. They will use a term T to mean one thing in their argument, then change T's meaning part-way through the argument without noting the change in meaning. This becomes apparent in many presuppositionalists use of Alvin Plantinga's modal OA. In this argument one can define God as the MGB (maximally great being) or the MEB (maximally excellent being). These definitions are not equivalent, as Plantinga explicitly notes. The MGB is defined as an MEB that exists in every possible world (i.e. an NE MEB), while the MEB is not defined as existing in every possible world. Of course, this does not stop some proponents of Plantinga's argument from equivocating between these two definitions. They will claim that in order to show the MGB is impossible, you must show the MEB is impossible; this is false. Or they claim that evidence for the existence of the MEB is evidence for the possibility of the MGB; this is also false.
Remember: equivocations are often difficult to spot. For example, a modal OA advocate might say, "the Kalam Cosmological argument at least shows that God is possible, which supports the modal OA's premise that God is metaphysically possible." This may seem like a harmless claim, until we ask the proponent to define what they mean by "God." Kalam has no bearing on the MGB since the Kalam does not make claims regarding every possible world. So if the proponent is to avoid an equivocation, they must claim "God" means "MEB" both times it is used. In that case, however, the Kalam has no bearing the Plantinga's OA, since Plantinga's argument needs the MGB to be possible, not just the MEB; there is all the difference in the world (both this world and other possible worlds) between saying "the MEB is possible" and "the MGB is possible." We can avoid difficult-to-spot equivocations by: 1) asking the MAG proponent to clearly define their terms and, 2) sticking to terms that have clear definitions. So I recommend asking the MAG proponent avoid the term "God" unless they explicitly define "God" and agree to stick to that definition, unless otherwise noted. Otherwise, just ask them to use the clearly defined terms "MEB," "MGB," etc. in order to prevent equivocations.
Asking for clear definitions also benefits MAG critics in other ways: apologists are notoriously vague about the meaning of terms such as "greater, better, perfection," etc. So if a MAG critic argues that greatness, perfection, etc. implies an impossible to instantiate property, or a property lacked by the theist's God, or some description which could apply to both the theist's God and another entity, etc. the MAG proponent can just switch to an ad hoc definition of greater, better, etc. that is tailor-made to avoid the critic's objection and have greatness, perfection, etc. only apply to God and only imply instantiable properties. I discuss this more in Ontological Arguments: A Critique. Expect the MAG proponent (or some apologists in general) to make the same "ad hoc red-definition" move regarding any vaguely defined term in any theistic argument. So it is always important to ask for clear definitions. Better to be safe than sorry.
Some apologists also tend to attach the label "God" to random stuff, simply to produce an argument for "God's" existence. For instance, a theist might use a PSR to argue that "God" is "the necessary thing which serves as the reason/explanation for contingent things." "Thing" here has no mental connotations: the thing could be a mind or a non-mental entity. So many, if not most, PSR proponents subtly rephrase the definition of “God” into "the necessary being which serves as the reason/explanation for contingent things," where “being” is meant to be read as "mental entity." This begs the question against atheists and pantheists who claim that the entity the PSR argues for could be a non-mental thing. The PSR argument does not show that the necessary entity is a mind; the PSR proponent need a separate argument for that conclusion. If you point this out to an apologist, expect them to respond that they always meant "being" to be read as neutral between "mind" and "not a mind." Of course, if you never pointed this out to them, they would have been happy to go on claiming the PSR implied a necessary mind existed. But if the PSR only shows that there is a "necessary thing which serves as the reason/explanation for contingent things" (call this thing N), why think N has thoughts, beliefs, etc. Why is the MAG advocate offering an argument for N, when we have no reason for thinking N is the God of traditional theism; i.e. a mind that communicates with people, has thoughts, etc.? To compensate, many theists resort to very poor supplemental arguments to show that N is a mind (see my exahange with Mentat1231). These arguments are normally much weaker than the PSR itself. So when we ask theists to provide clear definitions for terms such as "God", we often discover that their arguments are much less plausible than they first appeared.
The apologetic tendency to label random things “God” arises even in non-MAG arguments. For instance, I have seen theists argue that since naturalistic evolution cannot explain the diversity of life (a patently false claim, by the way), whatever does explain the diversity of life, that thing is probably God. Theist who ran this argument 300 years ago would say, "since scientists lack an explanation for the diversity of life, whatever does explain the diversity of life, that thing is probably God." So 300 years later, we discovered that naturalistic evolution is probably God! This "God-of-the-gaps" reasoning results from apologists attaching the label "God" to random thing X, without showing that X has any of the other properties usually associated with God. So once you ask an apologist to define "God" in the context of their argument, you'll usually find they are arguing for something completely uninteresting or only very tangentially related to the version of God which most religions adhere to (i.e. a powerful mind that interacts with humans, created this universe for the purpose of containing life, is at least mostly good, has knowledge far surpassing any other existent being, etc.; of course, all of these properties could apply to aliens, so I offer these conditions as necessary, but not jointly sufficient, conditions for God).
Second, if you are not familiar with the presented MAG or you cannot quickly recognize the flaws in the argument, then politely withdraw from the conversation and do some reading on the argument and its underpinnings. Nothing is more exasperating than dealing with someone who has no idea what they’re talking about and act as if they do (*cough* InspiringPhilosophy and MessianicDrew *cough*). Do not be that person. There is a huge difference between a person making a subtle error relating to something they are knowledgeable about (i.e. Plantinga’s mistakes in his presentation of the modal OA), and an uninformed person making absurd claims that would be fixed by perusing Wikipedia (i.e. every proponent of the modal OA who claims the argument proves God exists). However, don't be too depressed if you need to do some reading; most MAGs employ similar concepts and make similar mistakes. So once you've become familiar with some basic terminology such as modality, analyticity, question-begging, parody arguments, etc., you will be able to spot a MAG’s errors relatively quickly.
Third, ask the MAG proponent to explain the type of modality involved in their argument. Is this physical modality, epistemic modality, logical modality, broadly logical (or metaphysical) modality, etc.? Many, if not most, MAG proponents confuse the latter two. Logical modality relates to conceptual coherence and incoherence, while broadly modality relates to instantiability or actualization. William Lane Craig, disingenuous man that he is, states that broadly logical modality differs from logical modality while simultaneously associating broadly logical impossibility with incoherence and broadly logical possibility with coherence. His error has been explained to him before; he simply does not care. As long as his audience lacks the knowledge necessary to spot his mistake and correct him, he will continue to spout the same falsehood. And once he's among competent philosophers, he'll simply note that broadly logical impossibility differs from logical impossibility without daring to associate the former with incoherence, lest his claim be ripped to shreds. Since many MAG proponents do not understand modality, Craig is able to hoodwink them into confusing broadly logical modality with coherence.
But surely if we explain the difference between logical and broadly logical modality, show why almost every philosopher (or really: every modal philosopher I know of, including Plantinga) either reduces broadly logical modality to logical modality or distinguishes coherence/incoherence from broadly logically possible/broadly logically impossible (never both) [this is not an argument from the authority since we can cite the arguments these philosophers use to reach their conclusions; Craig provides no arguments for his ludicrous position], etc. modal OA proponents will see the error in Craig's ways? Well...no. Some modal OA proponents will simply ignore the logical/broadly logical distinction and demand that you show them the incoherence in the definition of "MGB" even though the MGB includes broadly logical, not logical, necessity. So some people will follow Craig no matter what he says without bothering to actually provide arguments for what he says or addressing critiques of his claims. After all, thinking for oneself is hard. However, you should still ask the MAG proponent what type of modality they are employing so you can hold them to that choice later.
Fourth, and finally, ask the MAG proponent to justify their choice of modality. As both I and Rayndeon note (see "Ontological Arguments: A Critique" and Rayndeon's blog), necessity is not granted by fiat. One does not just get to include necessity in one's being's description and expect the rest of us to take it seriously. Nor does one just get to claim that something is logically necessary or metaphysically necessary or epistemically impossible without justifying this modal claim. So if a PSR proponent claims God is logically necessary, ask them where the contradiction is in denying God's existence. Or if a modal OA proponent claims that God is metaphysically necessary, ask them what metaphysical absurdity (or unactualizable/uninstantiable situation) results from denying God's existence. You'll soon realize that most MAG proponents include necessity in their definitions of God for the sole purpose of running MAGs. They either: 1) have no justification for including necessity in God's existence, 2) do not understand the relevant modality well enough to provide a justification, or 3) their justification would work for a myriad of other beings; a fact which they usually avoid by engaging in special pleading.
Hopefully if you follow these guidelines in your encounters with MAG proponents, you can lead them to see the error in their arguments. And who knows, a proponent could address all of these questions while presenting a sound, non-question-begging MAG? But based on my personal experience, I doubt it.