Hence our verdict on these reformulated versions of St. Anselm's [ontological] argument must be as follows. They cannot, perhaps, be said to prove or establish their conclusion. But since it is rational to accept their central premiss, they do show that it is rational to accept that conclusion. And perhaps that is all that can be expected of any such argument.
- Alvin Plantinga, The Nature of Necessity (pg. 221)
So Plantinga himself, the author of the most popular modern ontological argument, thinks that all ontological arguments fail to establish theism. It's very strange, then, that so many theists (especially those you bump into on online phil. of religion forums) are so aggressive with his argument, many thinking it the best thing to come out of natural theology.
I like to classify arguments by their purpose: successful offensive arguments will convince a non-believer, while successful defensive arguments will rationalize a believer. For example I don't think there are any successful offensive arguments for moral realism; I don't think there's any argument that would make someone obligated to be a moral realist. But I do think there are arguments that rationalize moral realism, that show it to be something we can rationally believe or something that is not irrational to believe. Plantinga seems to be saying that his ontological argument, if successful at all, is successful as a defensive argument. I don't agree with him, I don't actually think his argument even goes that far for reasons I've explained elsewhere. Interestingly enough, though, if his argument were successfull in any sense, it could easily be modified to be a successful argument for moral realism as well. Moral truths, if they're true at all, are necessarily true (and moral facts, if they exist at all, necessarily exist), so Plantinga's ontological argument would pose a serious threat to any theist who thinks secular ethics is in some way irrational. At the very least they would have to admit that the ontological argument supports God's existence just as well as it supports moral realism.