The purpose of this paper is to examine several aspects of the relationship between Christianity and the rise of the new rationalistic spirit of the eighteenth century. It is in this connection that we intend to examine the thought of the French Huguenot preacher Jacques Saurin (1677–1730). Historians have held that the two leading ideas of that century, Nature and Reason, derive their meaning from the natural sciences. Such a point of view tends to ignore the greater realism towards nature and politics which developed within the Christian theological framework itself. From the sixteenth century on, we find orthodox theologians emphasizing the need for dealing with the world on its own terms. It was not so much the new sciences but rather the conflicts of the Reformation which brought out this increasingly rational attitude on the part of both Protestant and Catholic theologians. This development went on side by side with that secularized idea of reason which is of specific scientific inspiration. The means which theologians used to make room for a greater realism in their Christian framework of thought was casuistic divinity.