This paper presents the special issue on Formal Approaches to the Ontological Argument and briefly introduces the ontological argument from the standpoint of logic and philosophy of religion (more specifically the debate on the rationality of theistic belief). Arguments for and against the existence of God have been proposed and subjected to logical analysis in different periods of the history of philosophy. In an important sense, they all deal with the rationality of theist belief. Providing a good argument for the conclusion that God does exist, or that it is highly probable that he exists, might be a pretty strong case for the thesis that belief in his existence is rational. Similarly, a good argument for the conclusion that God does not exist could be said to support the thesis that theistic belief is irrational. A more basic approach than that would be to analyze the very concept of God. Can God create a stone so heavy that he cannot lift? If we say yes, then there is something God cannot do, namely to create such a stone; if we say no, there is also something he cannot do, namely to lift the stone. In either case he is not omnipotent. If really unsolvable, paradoxes like this (this is the paradox of the stone) show that the concept of God (who is, besides other things, omnipotent 1) is incoherent or contradictory. Like the concept of a squared circle, it could never be 1 For more on the concept of omnipotence and the paradox of the stone see [6].