“If we assume that everyone’s reason must or at least could attain the concept of God without an act of revelation, this could clearly happen only if guided by causality; and this is obvious enough not to require any proof. Even Christian Wolff says (General Cosmology,b preface, p. 1): ‘In 604 natural theology, we logically prove the existence of the highest being from cosmological principles. The contingency of the universe and of the order of nature together with the impossibility of (pure) chance are the steps one uses to climb from this visible world to God.’c And Leibniz before him had already said, with reference to the law of causality: ‘We could never prove the existence of God without this great principle’d (Theodicy, §44).
And likewise in his argument with Clarke, § 126: ‘I will go so far as to say that the existence of God could never be proven without this great principle.’e,116 By contrast, the thought developed in this chapter is so far from being essential and necessary to reason that it can instead be regarded as a proper specimen117 of the monstrous products of an age that was led by peculiar circumstances into the strangest deviations and nonsense. This was the age of scholasticism, which is without equal in the history of the world and can never return. At its apex, this scholasticism certainly produced the principal proof for the existence of God from the concept of the ens realissimum, and used the other proofs only secondarily, as accessories. But this is just a method of teaching, and does not prove anything about the origin of theology in the human mind. Here, Kant took the procedure of the scholastics for a procedure of reason, which is something he does quite frequently.”
Arthur Schopenhauer – The World as Will and Representation Volume I