I recently read Enlightenment Britain and the Creation of the Modern World by Roy Porter.  It was interesting to see the origins of some the ideas we now hold dear.  It seems to me that modern Paganisms are empiricist, whereas revealed religions are a priori and essentialist or systematic.  This would go a long way towards explaining the lack of any formalised theological system in modern Paganisms.

Apparently (p 56), the idea of the pineal gland as "the seat of the soul" comes from Descartes.  It was part of his attempt to separate the body from the soul (orthodox Christian thought had insisted on the unity of body and soul, because of the doctrine of bodily resurrection).  Descartes' insistence that body and soul were separate represented a mechanistic view of the universe; he also didn't believe that animals had consciousness.

The phrase "the Book of Nature" comes from early science, and the writings of Francis Bacon in particular (circa 1620).  In 1605, Bacon (as Lord Chancellor) separated science from theology in order to get round the problem of churchmen who were wary of prying into God's secrets.  Bacon said that theologians studied the Bible, whereas natural philosophers studied the Book of Nature.  (p 56)

The idea of the priesthood of all believers is a Protestant idea which has fed directly into modern Paganisms, and which was strongly influenced by Locke's empiricism.  Locke said that the mind matures from ignorance (the tabula rasa) to knowledge by gaining practical experience through the senses.  His

ideas of rational self-responsibility tie in nicely with the idea of direct contact with the divine.  (pp 69-71)

Another similarity is the belief in formulating your own belief system.

"every man in this enlightened age (having been fully instructed by those genteel and easy conveyances of knowledge, newspapers and magazines..." {presumed to have} ... "the liberty of making a philosophy (and I might add indeed a religion) for himself."  (Alexander Catcott)

Indeed, it was during the Enlightenment that some early advocates of a return to Paganism began to make themselves heard, including Erasmus Darwin (who wrote about Linnaeus' sexual system of plant classification in terms of classical mythology, and believed that sex was the basic principle of life), William Hamilton (excavator of Pompeii and Herculaneum), and Richard Payne-Knight (a friend of Josiah Wedgwood and William Hamilton's publisher).  (p 273)