Here's an interesting read to develop your argument from.
The most powerful version of the ontological argument, in my opinion, is presented in the book God and Necessity by Stephen E. Parrish.
The argument goes as follows:
1) The concept of the Greatest Possible Being (GPB) is coherent (and thus broadly logically possible).
2) Necessarily, a being who is the GPB is necessarily existent, and would have (at least) omnipotence, omniscience, and moral perfection essentially.
3) If the concept of the GPB is coherent, then it exists in all possible worlds.
4) But if it exists in all possible worlds, then it exists in the actual world.
5) The GPB exists (Parrish, 82)
This argument is also deductively valid. Premise 1) argues that the Greatest Possible Being is coherent–that is, there is no logical contradiction within such a being. 2) further defines what a GPB would be (Plantinga’s argument outlines this thoroughly). Premise 3) states the major part of the argument in a different way. Rather than arguing that it is possible that “maximal greatness” is exemplified in some possible world, Parrish argues that the concept of the GPB entails logical necessity along with such maximal greatness, and thus 3) follows from the previous premises, just as Plantinga’s version of the argument does. The key is to remember that in Parrish’s version of the argument, the coherence of the GPB is what is important, not the possibility (for if it is coherent, it is possible). 4) This is tautologically true. 5) follows from the previous premises.
What Parrish does here is actually takes out the possibility of denying premise 1) in Plantinga’s argument. Let’s look into this closely. Parrish argues that the concept of the Greatest Possible Being is coherent. Why is this so important? Well, because if we grant for a moment that the GPB exists, such a being could not fail to exist due to some kind of chance mistake or having some other being or thing prevent the GPB’s existence (Parrish, 105). The first point (that chance could not prevent the GPB’s existence) is true because the GPB would be logically necessary (it would either exist or not exist in all possible worlds). This claim is reinforced by the idea of maximal greatness being a universal property (above). The second point (nothing else could prevent the GPB’s existence) seems quite obvious. If there were a being or body or thing, etc. that could prevent the GPB’s existence, the GPB would clearly not be the Greatest Possible Being. If some other being were powerful enough to prevent the GPB’s existence, then that being would be greater.
So the only thing that could prevent the GPB from existing is self-contradiction within the concept.
Why is this? Well, after a little investigation it seems pretty clear. If the GPB is a coherent (and logically possible) concept, then such a being does exist. Let us say that the GPB is coherent. Let us then take some world, W, and see whether the GPB can fail to exist.
The concept of the GPB includes logical necessity in all possible worlds. The GPB has all the properties of maximal greatness. This means that these properties are universals. We can simply refer back to the argument above. If the GPB exists and has omnipotence, omniscience, etc. then it must exist universally, because, again, if some being is the GPB in only 200/1,000,000,000 possible worlds, the being that is GPB in 2,000 is greater. But this seems ridiculous, for the truly Greatest Possible Being must exist in all of them, for if there was a possibility for some being to exist in all the worlds that the GPB exists in +1 and exemplify the maximally great attributes, then that being would be the GPB (and the previous one would not really have omniscience, etc., for the GPB would be more powerful, existing in all possible worlds, and being sovereign in all possible worlds) . Now let us return to W. It now seems completely clear that W could not be such that, if the GPB is coherent (and therefore possible), W could not fail to exemplify the GPB.
But have we then demonstrated that coherence is really the issue here? Is it possible that we are just thinking up some thing, calling it the GPB, and then arguing it into “supposed” existence? Logically, it does not seem so.
The reason is because we are arguing that the GPB entails these properties. Things have, essentially properties. I exemplify the property of “having fingers.” I also exemplify the properties of “being finite,” “being human,” “having two feet,” etc. These properties don’t belong to me simply because someone sat around and decided to assign them to me, rather they belong to me because of the kind of thing I am. (Parrish argues similarly, 55). But in the same way, we could answer such objections by saying that these properties are part of the concept of God because that’s the kind of thing God is. Certainly, there have been all kinds of “gods” claimed throughout history that are finite in power or activity, but those aren’t the “gods” whose existence we are arguing for. Rather, we are arguing for the existence of the God of classical theism, and that God has such properties as necessary existence (in the analytic sense), omnipotence, omniscience, etc. This objection really doesn’t have any weight. But again, let’s assume for the sake of argument that it does.
Let’s assume that the objection may be true. We are just taking some “X” and arbitrarily saying that it is omnipotent, necessary, etc. Does that preclude such an object existing? I don’t see how this could be true. But even further, some claim that this doesn’t match up with Christianity’s concept of God. This seems preposterous. One needs only to open a Bible to find that, while words like “omnipotence” are not used, words like “Almighty”, “Most High”, and the like constantly are. And what kind of objection is this really? Is the person making this objection going to concede that it is possible that there exists some nearly-omnipotent-but-not-actually-omnipotent creator of the universe? No, the objection is beyond logic and into emotional repugnance at the thought of God actually existing.
But we can even go further. For let us simply define God as the Greatest Possible being. This seems like it could very easily operate as a definition of what “God” is, at least on classical theism. Well then, what properties might this Greatest Possible Being have? And then we simply build them up. Omnipotence seems obvious, as does omniscience, as does necessity, etc. So this isn’t some arbitrary assigning-to of properties, but rather such properties are part of the GPB simply because of what the GPB actually is, if the GPB existed.
Now we can return to the matter at hand. Does God exist? Well it follows from all of this that yes, God does exist. The theist has established that there are some arguments that deductively prove that God does exist. The only “way out” for the atheist is to attack premise one and argue that the concept of the GPB is, in fact, contradictory. And let’s be honest, there have been many attempts to do so. I can’t possibly go into all of them here, but I can state simply that I remain unconvinced. Often these arguments are things like “Omnipotence and omniscience are impossible to have, because if God knows in advance what He’s going to do, He can’t do anything else!” This argument is obviously false, for simply knowing what is going to happen is not causation. I know that a sheep is an animal, this does not cause the sheep to be an animal. I know that I am going to finish typing this post, that does not cause me to do so. Rather, I choose to continue typing and finish this post.
Of course, one might say “You can’t really know you’re going to finish this post! Your computer might explode and you may get brain washed, etc.” Well that is a whole different debate, but I think that such objections, ironically, actually apply not at all to God. For if God is omniscient and omnipotent, it seems clear that God actually would be above such things! For nothing could prevent God from finishing something He knows He’s going to do! Not only that, but God’s knowledge is such that He actually would know He is going to do something, and freely chooses to do so. I don’t see why God’s foreknowledge of an event somehow limits omnipotence, especially when one considers that God is part of agent-causation, so God chooses to do the things He is going to do. Thus, the argument falls apart.
But now I’m already farther off track than I was (and thus preventing myself from finishing this post, AH the irony!). Suffice to say that I very much doubt that any objection to the coherence of the GPB even comes close to succeeding. But then, if that is true, God exists.
Therefore God exists.