To reconstruct a critique of the ontological proof of the existence of God in Schelling’s philosophy I examine his interpretation of the ontological argument by Anselm of Canterbury and Descartes as well as Schelling’s assessment of the critique of the Kantian ontological proof of the existence of God. I propose a reconstruction of Schelling’s account of undoubted being which cannot be deduced from the concept of the totality of all that is possible and therefore must come before any thought. He interprets reason as having an ecstatic nature which posits precedent undoubted being. This enables Schelling to formulate his own version of the thesis on the unity of being and thought, whereby being comes first and thought is only second. Against this background I analyse Schelling’s interpretation of the Kantian account of the ideal of reason. Schelling, on the one hand, agrees with Kant that being is not a real predicate, hence real existence cannot be deduced from essence in the sense of “what.” But, on the other hand, in contrast to Kant, he believes that real existence of the individual absolute must be assumed, which would be the subject for all possible predicates and whose being is ecstatically posited by reason as being external to itself. I raise the question of the relevance of Schelling’s thought for modern ontology, above all in overcoming ontotheology. Proceeding from the works of J. F. Courtine and L. Tengelyi I single out two aspects of Schelling’s doctrine that are relevant to my subject: (1) the priority of existence over essence in God’s being and (2) the fundamental irreducibility of God to a necessarily existent being, i.e. God’s freedom. It is evident that, in his interpretation of Kant, Schelling somewhat simplifies his train of thought and leaves it unclear how Kant links the concepts of necessary being and the supremely perfect being. It is also evident that Schelling’s concepts of “contingency,” “contingent necessity,” “the whole experience” need further study.