This dissertation develops an original interpretation of the relationship between reason and religious belief in the work of Pierre Bayle, a seventeenth-century skeptic, that I call "philosophical fideism." The underdetermined, and often paradoxical, nature of Bayle's writing makes interpreting him a formidable task; I therefore begin by sketching out the contemporary interpretive landscape of Bayle studies, currently deeply divided over the issue of Bayle's conception of the reason-faith relationship. I subsequently examine other conceptions of the reason-faith relationship among rationalists and skeptics of the seventeenth century, and argue that Bayle's position on this issue is deeply influenced by his Cartesian inheritance. I argue that the central, but neglected, factor in understanding Bayle on the reason-faith question is the influence of seventeenth-century Calvinist rationalism, particularly that of Moïse Amyraut. I show how Amyraut's tripartite distinction of revealed truths provides the framework for a central element of Bayle's philosophical fideism, and argue that a small group of revealed truths that Bayle calls "the Christian mysteries" form the core of Bayle's philosophical fideism. I attribute a conception of reason to Bayle that I call "qualified Academic skepticism," in contrast to the "supersceptical" interpretation of Richard Popkin on the one hand, and the Stratonian interpretation of Gianluca Mori on the other. Finally, I explain the grounds of Bayle's claims about the erring conscience and the justification for religious toleration. Conscience plays a crucial role in Bayle's philosophical fideism, not only epistemologically, but also morally. The erring conscience supports the interpretation of Christian mysteries as Bayle's own religious first principles, greatly increasing the significance of Bayle's doctrine of the erring conscience. If the conscience is the source of one's core beliefs and of their moral force, then reason, though still able to examine critically the claims of conscience, would be impotent to mitigate the moral force of the duties and rights of conscience. One cannot in good conscience, therefore, be intolerant of those who articulate alternative "first principles" since their source is the individual conscience itself. This account thus establishes a ground for religious toleration that is independent of, but compatible with, religious first principles