There are other reasons, neither few nor weak, for which the dignity and majesty of Scripture are not only affirmed in godly hearts, but brilliantly vindicated against the wiles of its disparagers; yet of themselves these are not strong enough to provide a firm faith, until our Heavenly Father, revealing his majesty there, lifts reverence for Scripture beyond the realm of controversy. Therefore Scripture will ultimately suffice for a saving knowledge of God only when its certainty is founded upon the inward persuasion of the Holy Spirit. Indeed, these human testimonies which exist to confirm it will not be vain if, as secondary aids to our feebleness, they follow that chief and highest testimony. But those who wish to prove to unbelievers that Scripture is the Word of God are acting foolishly, for only by faith can this be known. Augustine therefore justly warns that godliness and peace of mind ought to come first if a man is to understand anything of such great matters.
Institutes of the Christian Religion, Volumes 1 & 2, ed. John T. McNeill, trans. Ford Lewis Battles, The Library of Christian Classics (Louisville, KY: Westminster John Knox Press, 2011) Vol. 1.8.12. p. 92.