For instance, Gaunilo argued that its possible to construct an argument in the exact same form as the ontological argument, that claims to prove the existence of the perfect island: this island must exist for if it did not then it would be possible to conceive of an island greater than that island than which no greater can be conceived, which is absurd. Therefore, if the ontological argument works, then the argument for the existence of the perfect island must work too. They are both logically similar, so they stand, or fall together.
However, the argument for the island is clearly spurious, as we have no reason for this perfect island to exist. Unless a theist can point to some relevant difference between his argument for the existence of God, and Gaunilos argument for the existence of the perfect island, the theist will have to abandon the ontological argument for the existence of God, as well as Gaunilos, because if one fails, then they both do, as they are so similar. This weakness of the ontological argument adds fuel to the fire for atheists through Gaunilo putting forward his argument for the existence of the perfect island as an objection to the ontological argument. He knew that they would both fall, which shows that neither argument works which further supports atheists.
Another weakness of the ontological argument which gives support to atheists is Immanuel Kants objection to it: Existence is not a Predicate. Fundamental to both Anselms and Descartes form of the ontological argument is that existence is a predicate, an attribute or a quality that can be possessed or lacked. These qualities for example, may or may not belong to a thing or being and their presence or absence is part of our understanding and apprehension of it. Kant observed however, that existence isnt associated with the definition of something, since it didnt add to our understanding of that thing.
This supports atheism as existence is merely something that we can have or not have, and it is absurd to say what something is like without it existing. It also supports atheism as an atheist would say that we cant ascribe existence a priori to a definition of a perfect being.
Bertrand Russell furthered Kants observations, proposing that existence was not a predicate, but a term used to solely indicate the instance of something in the spatio temporal world. To say something exists only tells us that that thing occupies a space on the earth. For example saying Cows are brown, and exist tells us only one thing: they are brown. The and exist shows that they are on the earth, however its a tautology since to say they are brown, effectively suggests that they exist and arent imaginary.
Thomas Aquinas had already questioned this aspect of the ontological argument, claiming that Anselm was guilty of making transitional error moving from the definition of God to the existence of Him. He observed that Anselm also was guilty of making an assumption about the definition of God that was not necessarily shared by all believers. This supports atheism because not all believers believed in Anselms definition of God, which poses the question, if people have different views on the right or wrong definition of God, whats to say that they could ALL be wrong, and atheism is the truth in it all? Furthermore, Aquinas said that understanding the term God means that God exists in the understanding, not in reality.
Gods existence in reality must be demonstrated a posteriori, with evidence or experience. Atheism is supported in this way, as atheists could argue that anyone can believe what they like in their minds, but something has to be evident in material form in reality to be actually real. Overall, the weaknesses generally give support to atheism as they show alternatives and flaws in the ontological argument. The ways in which atheists interpret the critics objections leads to further belief that God does not exist. Atheists are supported primarily by the fact that the ontological argument is a priori, so does not rely on empirical evidence, therefore if there was empirical evidence for Gods existence, they might be swayed slightly in their beliefs.