Knowing something of the impact of unbelief on Christian doctrine, especially as it has occurred over the last two hundred years, is far from being an academic exercise. The nature of the unbelief causing pressure changes (modernism and postmodernism), but the process of unbelief realigning doctrines around a different epistemology and locus of authority remains.
"Rationalism" never is the direct product of unbelief. It is the indirect product of unbelief, among men who would fain hold their Christian profession in the face of an onset of unbelief, which they feel too weak to withstand.B. B. Warfield, "The Latest Phase of Historical Rationalism," in The Works of Benjamin B. Warfield vol. IX Studies in Theology, p. 591
Rationalism is, therefore, always a movement within the Christian Church: and its adherents are characterized by an attempt to save what they hold to be the essence of Christianity, by clearing it from what they deem to be accretions, or by surrendering what they feel to be no longer defensible features of its current representations.
The name historically represents specifically that form of Christian thought which, under the pressure of eighteenth century deism, felt no longer able to maintain a Christianity that needed to appeal to other evidences of its truth than the human reason; and which, therefore, yielded to the enemy every element of Christian teaching which could not validate itself to the logical understanding on axiomatic grounds. The effect was to reduce Christianity to a "natural religion."