In this paper, I will argue that the modal ontological argument succeeds an alleged defeater in the form of a reductio ad absurdum. I have come across a reductio ad absurdum of the ontological argument (St. Anselm’s, anyway) that I have been thinking about for some time. Not really knowing what to do with it, I have thought of two ways out. So, I will lay out the modal ontological argument, the reductio, and then offer two ways out which, I think, minimally succeed in showing how the reductio, even if it is prima facie convincing, does not stand a closer look.
Here is the modal ontological argument as Alvin Plantinga formulates it:
- It is possible that a maximally great being exists.
- If it is possible that a maximally great being exists, then a maximally great being exists in some possible world.
- If a maximally great being exists in some possible world, then it exists in every possible world.
- If a maximally great being exists in every possible world, then it exists in the actual world.
- If a maximally great being exists in the actual world, then a maximally great being exists.
- Therefore, a maximally great being exists. 
The crucial premise is (1). For if one denies (1), the whole argument collapses. While even philosopher William Lane Craig admits that (1) is “admittedly very difficult” for the theist to defend, nonetheless I think there can be progress in this area. For instance, take three natural theological arguments: the cosmological argument, teleological and axiological. Even if one rejects all three of these arguments, there are many, many more. For instance, the cosmological argument in the form of a contingency argument, the argument from consciousness and the argument from love et cetera. The intellectually humble approach, I think, is for the non-theist to admit the metaphysical possibility of the existence of God. But, of course, one can always deny (1) for ad hoc reasons.
Here is the reductio:
- It is possible that a maximally bad being exists.
- If it is possible that a maximally bad being exists, then a maximally bad being exists in some possible world.
- If a maximally bad being exists in some possible world, then it exists in every possible world.
- If a maximally bad being exists in every possible world, then it exists in the actual world.
- If a maximally bad being exists in the actual world, then a maximally bad being exists.
- Therefore, a maximally bad being exists
So, in the place of ‘great’, ‘bad’ is substituted. This means that if a maximally great being exists, so does a maximally bad being (which might not be what the theists wants); the theist is then offered an option: Deny the ontological argument or hold it and admit that there exists a maximally bad being. This is an inclusive ‘or’, I should say in passing. And, so, the theist can no longer hold to the ontological argument, so the argument goes.
Here are two responses which I think sufficiently show that this is not so.
(1) The first premise in the reductio is not self-evidently or intuitively true. Consider the justification for (1). There simply isn’t any. Now, one might say that “any natural theological argument can successfully show that the “God” that exists is a “maximally bad being” since many of the arguments do not establish His moral properties.” But this is simply false. What, for instance, the Kalam argument gives you, is a concept of God without moral properties—granted; however, when taken conjunctively  with, say, the axiological (moral) argument, things are different, for then the argument does succeed—if true—in establishing (i) the existence of God and (ii) the existence of a moral God. To deny (1) of the reductio can be established on two grounds, though: (i) Deny the coherence of a ‘maximally bad being’ or (ii) show how it is improbable and therefore warranted to reject (1). I do not want to argue as the defender of (i) would want to. For the sake of argument, I will accept (i). But, (ii) seems more promising. Given the plethora of natural theological arguments, (1) in the modal ontological argument seems at least minimally plausible to accept. But, in the reductio, there is simply no reason to accept (1) (asides “I’ll just grant you (1)” reasons).
(2) Take an analogy. Suppose that instead of “maximally bad being” we say “maximally bad book” (i.e., Richard Dawkin’s The God Delusion). While the latter is incoherent, it also does not have the great making property of “existing.” Applied to the reductio, “existing” seems to be a great making property—not a bad one. If one says in defense of (1) that “a bad being would be worse if the being existed”, they do not realize that “existing” is a great making property—and therefore if the “maximally bad being” exists, it is not really a “maximally bad being.” Why? There is a worse being, a “maximally bad being that does not”. So, we have a reductio of the reductio: the “maximally bad being”, in order to exist, has to fail to exist. The law of non-contradiction has it that P & not-P is logically impossible and therefore either the being exists or does not; in order to exist, it must fail to exist, and therefore the reductio is a reductio of itself.
These are just two reasons I see out of the argument, and I hope I have shown that the reductio does not succeed as a successful defeater of the modal ontological argument.
 http://www.reasonablefaith.org/misunderstanding-the-ontological-argument Accessed August 7th, 2016.
 Natural theological arguments taken conjunctively can raise the probability of God’s existence immensely, as Timothy McGrew, William Lane Craig and Alvin Plantinga argue.