Harrison (‘Religion’ and the Religions in the English Enlightenment, 12-13) argues that the Platonic revival of the Renaissance was one of the key sources for the modern notion of “religion.” The point is clearest in Ficino:
“In De Christiana Religione (1474), he proposed that religio, despite appearances, was at all times and in all places the same thing – namely, a universal property of humanity, a distinguishing characteristic, and the one thing which made humanity human: ‘The word of god is as natural to men, as is neighing to horses or barking to dogs.’ We should not be deceived by the appearances of multiformity, urged Ficino: ‘All human opinions, all responses, all customs change – except religion.’”
Religio no longer named differings sets of rites, ways of life, and customs, as it did in the Middle Ages, when it often referred to particular religious orders. Instead, it referred to a putative ineffable something that lay behind the multitude of particular practices. Religion is whatever is left when religions are stripped of their liturgical “accidents.”
This de-historicized understanding of religion makes it possible to construct a theory of religion while ignoring the facts of religious borrowings and influence. Because religion is a natural human instinct that is everywhere the same, there is no need to give an historical explanation for the similarities or differences between religions.