"Let us build such a church, that those who come after us shall take us for madmen," said the old canon of Seville, when the great cathedral was planned. Perhaps through every mind passes some such thought, when it first entertains the design of a great and seemingly impossible action, the end of which it dimly foresees. This divine madness enters more or less into all our noblest undertakings.

Here Longfellow is translating or paraphrasing an expression attributed to a canon of Seville, also quoted as "we shall have a church so great and of such a kind that those who see it built will think we were mad". - Table-Talk (1857)