Faith can be described as a strong belief in something and is often associated with spiritual conviction not necessarily requiring strong evidence or proof. Because of this, not only would some scholars argue against the idea that faith should be based on reason alone but even go as far as to suggest that faith is fundamentally at odds with reason. Kierkegaard argued that reason only provides a probability that faith is justified and is prone to frequent revisions undermining the idea of the ‘passional commitment’ that faith requires.
Furthermore, given that the passion argument attributes a greater reward to higher levels of risk then the increased probability provided by reason would actually diminish the value of faith. However, it could be argued that devoid of logical reasoning faith is reliant on a believer’s own strength to maintain it rather than reason would should hold true in spite of this. This would suggest that a faith based on reason would be less subject to factors such as emotions.
That said, this idea would likely be challenged by Barth who argued that revelation alone should be the basis of a faith position echoing Kierkegaard’s sentiments with regard to the adverse effect of reason on faith. Barth’s propositional approach to revelation advocates against reason supporting faith due to his belief that human reason was corrupted by the fall, whereas revelation is the direct words of God. This undermines the suggestion that faith should be based on reason alone if the reason it is based on is corrupted.
Nevertheless, Aquinas outlined the value of reason in faith appearing to challenge Barth’s position. His development of a Cosmological argument using reason goes some way to indicating that belief at least in God’s existence was possible through reason. That said though Aquinas saw the value of reason he did not suggest that faith should be based on reason alone acknowledging that concepts such as the Trinity could not be understood through reason and also placed reasoned Natural and Human Law below revelation in his four levels of Law.
Additionally his reaction to a revelation he received in 1273 which led him to described his writings (based on reason) as “like straw” potentially implying that he came to the conclusion that revelation is superior to reason further reinforcing the importance of revelation in faith rather than just reason alone. Clifford in contrast argues that it “is unethical to believe anything on the basis of insufficient evidence” emphasizing the role of reason in faith and the irresponsibility of having a faith position not based on reason.
That said, there is clearly a strong degree of subjectivity in what ‘sufficient evidence’ is with different people potentially having differing criteria for what evidence is enough to believe something. This highlights the fact that different people may reason in different ways whereas faith based on propositional revelation should not have the same degree of subjectivity.
That said, non-propositional scholars such as Rahner would argue that in order to understand revelation reason is required however like Aquinas this requires reason in partnership with revelation rather than reason alone. To conclude, reason potentially has some degree of value in faith given that it isn’t prone to the subjectivity of revelation, however it’s possible that it may only be able to lead to fides (belief that) rather than fiducia (belief in) and consequently may not be sufficient for an I - Thou relationship with God.
As a result it is arguably doubtful whether a reasoned argument for God would be sufficient to bring about long-lasting faith, especially given its requirement for commitment as posited by Kierkegaard. Because of this it seems fair to conclude that reason alone is not sufficient for faith requiring support from areas such as revelation.