One of most surprising aspects of the shift from scholastic natural philosophy to the new mechanist natural philosophies in the early decades of the seventeenth century is the retention of a doctrinal conception of knowledge. There was an assumption not only among scholastics, but also among many of their seventeenth-century opponents, that philosophy—and especially natural philosophy—had to take a doctrinal form. This is despite the fact that many of the considerations that motivated this view among the scholastics were rejected or ignored by their modernist opponents. The central argument of this essay is that doctrine is not something at the core of religions in general, but rather something of specific concern to Christianity. In looking at how this specific concern informs the development of natural philosophy in the seventeenth century, it emerges that it is not a one-way process: in entering into a symbiotic relation, Christianity and natural philosophy are both affected. Christianity becomes largely reduced to its cognitive content, becoming both more science-like and more focused on its doctrinal core, while at the same time the cognitive content of natural philosophy, already somewhat doctrinal in nature, now has this reinforced, as it becomes more like Christianity in its aspirations.