A Catholic defence of reason does not identify faith with reasoning
They are both methods of proof which cannot themselves be proved.
A useful contemporary comparison is Vatican I's Dei Filius, promulgated 24 April 1870, inspected in the Vatican's online Latin text. Chapter IV explicitly says that faith protects reason from error and that the two assist each other. Chesterton's defence of religious authority thus has a recognizable Catholic analogue. Dei Filius, chapters II–IV.
But the council distinguishes two orders of knowledge by both their principle and their object: natural reason and divine faith are not interchangeable procedures. Chapter II affirms that God can be known with certainty through created things by natural reason; chapter III grounds supernatural faith in God's revealing authority, while also invoking external signs of revelation's credibility.
Chesterton's shared ‘unprovability’ is therefore best read as his philosophical comparison between ultimate commitments, rather than a complete account of Catholic teaching about evidence. The council supports a claim that faith can guard reason; it does not establish his sweeping historical claim that crusades and persecutions were originally organized for that purpose. Nor does this comparison demonstrate that Chesterton was borrowing directly from the document.
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