Excerpted from Epistemics of Divine Reality (2009, 2011), pp.105-107
Kant resolutely argues that the traditional arguments for the existence of God, viz. the ontological, the cosmological, and the physico-theological (teleological) arguments are based on false premises. They proceed from the false assumption that quantity, quality, relation, and modality are inherent in the universe and not merely subjective to the knower alone. The arguments against the arguments for the existence of God are as follows:
a. The Ontological Argument: The ontological argument of St. Anselm (1033-1109) proceeded from the assumption that God was ‘that than which a greater cannot be conceived.’ However, if this God did not exist then everything conceived of would be greater than the conception of God for reality is greater than an idea. Therefore, God as ‘that than which a greater cannot be conceived’ must of necessity exist. Rene Descartes had his own form of the ontological argument in which he argued that since God is by definition the supremely perfect being, He cannot lack existence, for that would mean that He was not a supremely perfect being; and since existence is a necessary attribute of perfection, God exists necessarily.
Kant argues that though the inference from contingent existence to necessary existence is correct and unavoidable, the conditions of the understanding refuse to aid us in forming any conception of such a being. Thus, the ontological argument is correct as far as words are concerned; but when it comes to actually forming a concept of the absolute and necessary being the argument fails. Further, the argument rests on judgments alone and cannot thereby alone establish the reality of anything. In Kant’s own words: ‘the unconditioned necessity of judgments is not the same as an absolute necessity of things.’ Alluding to Descartes’ analogy of the triangle Kant writes that though to posit a triangle and yet reject its three angles would be self-contradictory, there is no contradiction in rejecting the triangle with its three angles together. To put it the other way, if suppose in the analytical statement, ‘all bachelors are unmarried men’ the subject ‘bachelors’ implied the predicate ‘unmarried men,’ it still does not conclusively prove that there really are unmarried men or bachelors in the world. The statement is just a verbal one and is not corroborated by empirical evidence. In the same manner, though the subject ‘the supremely perfect being’ implies the predicate ‘has existence as an attribute,’ yet it does not conclusively prove that there really is a supremely perfect being in accordance to the words. One can reject both the subject and predicate and still commit no contradiction. In addition, all existential propositions (that declare the existence or non-existence of the subject) are synthetic and not analytic and, therefore, the rejection of the predicated would never be a contradiction: ‘all bachelors are unmarried men’ is not the same as ‘all bachelors exist.’ On the other hand if existence was to be considered as an attribute of anything, it is clear that this could not be true since an attribute adds to something and thus modifies it, but to say that something is does not really add anything to it. ‘The small word “is” adds no new predicate, but only serves to posit the predicate in its relation to the subject.’  Therefore, existence cannot be an attribute. Even grammatically, it is understood that the words ‘is’ and ‘exists’ are not adjectives but verbs.
However, even more difficult is the attribution of existence to an idea having a priori and not a posteriori status. Kant says:
Whatever, therefore, and however much, our concept of an object may contain, we must go outside it, if we are to ascribe existence to the object. In the case of objects of the senses, this takes place through their connection with some one of our perceptions, in accordance with empirical laws. But in dealing with objects of pure thought, we have no means whatsoever of knowing their existence, since it would have to be known in a completely a priori manner. Our consciousness of all existence (whether immediately through perception, or mediately through inferences which connect something with perception) belongs exclusively to the unity of experience; any [alleged] existence outside this field, while not indeed such as we can declare to be absolutely impossible, is of the nature of an assumption which we can never be in a position to justify.
Thus, since the idea of God as a perfect being cannot be empirically justified, it is impossible to certify whether such a perfect being exists or not in reality. Here it may seem that Kant is leaning towards empiricism, but it must be noted that he is only saying that necessity and strict universality can only be applied to that which is a priori and, thus, to the forms of intuition and the categories of thought alone. To extend these to anything beyond these is to go beyond justification. One can be sure that the statement ‘every cause has an effect’ is true since causality itself is a category of the mind and cannot be thought off. However, the same cannot be said of the existence God or any other being in the world. The distinction between the a priori constituents of the mind and the a posteriori world of senses once understood, the ontological argument cannot stand any longer. Thus, the ontological argument is dismissed.
 “Ontological Arguments,” Standford Encyclopedia of Philosophy (http://plato.stanford.edu/entries/ontological-arguments) The Critique of Pure Reason (trans. J. M. D. Meiklejohn; internet edition) The Critique of Pure Reason (trans. N. K. Smith), p. 501 That as the three angles are integral to the conception of a triangle, existence is integral to the conception of perfection. “supremely perfect being” are just words and have no accompanying conception. The Critique of Pure Reason (trans. N. K. Smith), p. 504 Ibid, p. 505 Ibid, p. 506