If I understood the ontological argument correctly it goes something like this:

I conceive of a most perfect being. Now, if that being did not actually exist, then a more perfect being would be one that actually existed, so therefore, since I actually conceived of a most perfect being, it must be one that actually exists.

Anyway, although a theist myself, and although I find this argument quite interesting, I am trying to see if it holds, so one counter argument that I am thinking of is "Ok, but how do you know that you actually do, or can, conceive of a most perfect being?"

It seems to me, namely, that the whole argument depends on that we actually do, or can, conceive of a most perfect being. So that must first be proven or established, that we do, or can, do that.

And of course, if an individual is totally convinced that he actually does conceive of a most perfect being, then yes, I guess the ontological argument must make him accept that being as really existing.

But who can say they have that ability to conceive this?

To personally answer that last question, well, sometimes I actually think I can, by the grace of God, and at those times the ontological argument is inspiring.

So, in a way, it seems that the argument is urging me to try, and if I succeed then good, otherwise, too bad. It is trying to lead me to look from a certain perspective, to see a hopefully self-evident truth. And whether I can see it or not depends on my ability to see at that moment, which in turn would of course also depend on the will of God, provided God exists. This last thing relates a bit to my argument about agnosticism.