Even though we have now concluded the presentation of the modal ontological argument for God’s existence, it is important to also answer objections to the argument. Three of the most common objections will be answered below.
1. I Don’t Agree with Your Great Making Properties
Imagine, as you were about to eat lunch, your friend urgently commanded you not to take a single bite of food. He then proceeds to give you an argument intended to convince you that your food has been contaminated with a particular kind of poison. Let’s say you accept his reasoning, but you conclude that it is contaminated with an entirely different kind of poison. Would you still eat the food? Hardly! Even though you believe it is a different kind of poison, you are still committed to the same conclusion. Namely, don’t eat the food!
A very similar situation results when thinking about a maximally great being. It is certainly possible that many people will disagree with various characterizations of maximal greatness. It is certainly possible that my own brief analysis could be improved. These disagreements, however, are largely irrelevant to the force of the argument. One might even go so far as to reject the God of the Bible as being morally perfect, as many atheists do. Regardless, each person still has some idea of what moral perfection is. And if you believe that your version of perfection is possibly exemplified, just as you personally believed your food had been poisoned, the conclusion follows.
Therefore, the only relevant question becomes, “do you believe that maximal greatness is possibly exemplified?” If premise 1 is believed to be true, whether your characterization of maximal greatness is identical to someone else’s or not, then premises 2-6 follow with relative ease. Therefore, the argument can be reduced to a conditional: If you believe that God’s existence is possible, then you must believe that God exists.  To the contrary, to deny God’s existence, you must deny that God’s existence is even possible. And that is a difficult task indeed.
2.Then Maximally Great Islands and Computers Must Exist, Too.
Parody of this argument is by far the most common form of objection. Since the possible existence of a maximally great being is said to imply the actual existence of that being, would not the possibility of other maximally great things also imply their existence? For example, the objector could say that a maximally great island or a maximally great computer exists in some possible world. Given the definition of maximal greatness, the same argument which brings God into existence also brings islands and computers (and unicorns) into existence. But this seems ridiculous! Either the reasoning that demonstrates God’s existence is also ridiculous, or there is some flaw with these parodies.
When dealing with most parodies, there are at least two major flaws associated with them: First, the great-making properties are not obvious. Second, the properties do not have maximum values.
- First, the great-making properties of islands and computers are not obvious. What does the greatest island look like? Is it remote and barren, or local and full of resorts? Is it large or small, and how large or small is greatest? Similarly, what would the greatest computer look like? Would it be portable or stationary? Apple or PC (perhaps Linux)? Aluminum, titanium, or gold casing? How tactile should the keyboard feel?
- Second, any great-making properties that are agreed on do not have maximum values. If it is agreed that palm trees and resorts are great-making for islands, how many of them should the island have? There could always be one more to make the island “greater.” How many pixels are on the screen of the greatest computer? How loud would the computer’s speakers be? How much memory would it have? How fast is the processor? There is no maximum value, and you can therefore never reach the “greatest” of any of these. 
For these two reasons, popular parodies fail. The concept of a maximally great island or computer is not actually possible. In contrast, God’s great-making properties are obvious, and they do have maximum values, and so it is possible for Him to be the “maximally great” Being. Other problems exist with attempted parodies, but these two seem sufficient to demonstrate the point.
3.What About the Existence of a Quasi-Maximally Great Being?
If a maximally great being is possible, what prevents a quasi-maximally great being from also being possible? That is, wouldn’t it also be possible for a being to possess all the great making properties of the maximally great being, except to have imperfect knowledge of some random proposition? Maybe this being does not know when Abraham Lincoln was assassinated. After all, such a being would not be incoherent, and its existence seems just as possible as the maximally great being’s. Does this mean that any number of quasi-maximally great beings could exist, each one being ignorant of a single proposition that the others knew? This does not seem to be true.
Remember, maximal greatness includes omnipotence. Omnipotence implies that everything that exists would be subject to that being’s power – that nothing could exist independently of that being’s power.   Therefore, the maximally great being would have to be able to freely choose not to create this quasi-being in any number of possible worlds, implying that such quasi-beings would hardly be necessary. Therefore, maximal-greatness is logically incompatible with quasi-maximal greatness. 
In conclusion, the modal ontological argument for God’s existence is first logically valid. Second, the premises are true, and the argument is therefore sound. Third, various objections have been answered. As said in the beginning, this argument is not for the philosophically faint of heart. And because of its difficulty, this argument is dismissively thought to be ineffective. But those who study it will often find it captivating, and a few may even find it compelling. The Ontological argument for the existence of God is indeed powerful.
 Craig, William Lane. Reasonable Faith: Christian Truth and Apologetics, third edition, p. 169.
 Ibid., p.186.
 Ferré, Frederick. Basic Modern Philosophy of Religion, p. 123.
 To examine the Christian view of God’s omnipotence, see Col. 1:16-17; 1 Co. 8:6; Rom. 11:36; Acts 17:25; 2 Pe. 3:5-7; etc.
 Craig, William Lane. Reasonable Faith: Christian Truth and Apologetics, third edition, p. 186.