Protestantism is the daughter of Renaissance Humanism and the midwife of Enlightenment philosophy. In that time especially, men began to place their own reason above the divine authority of the Church.
A long time ago as a presuppositionalist Presbyterian I realized that like everyone else my thinking was suffused with unexamined presuppositions. I decided that I wanted to change that, investigate them, and root out those that were unbiblical. It was tough sledding for me for a long time: how do you think about why you think what you think, when there are things that affect how you think that you have never thought about? As I recall this was a project of mine for a fruitless year or two (off and on) before I finally made headway in realizing the very thing that Bryan affirms in the quotation above.
It is absurd to suppose that the Protestant Reformation took place in a philosophical clean room, unstained by the prevailing philosophical winds. It is all the more absurd when we consider the resemblance between the principle of humanism—man as the measure of all things—and the principle of the supremacy of individual judgment within Protestantism. The Protestant reserves the right to interpret Scripture himself, and to judge the truth of any proposition according to his own understanding of the Bible. He determines for himself what the Bible says—in other words, he decides for himself what God says—and he determines for himself whether his congregation or denomination is in harmony with that. This is obviously indistinguishable from humanism in terms of its operation: he has made himself the measure of religious things.
It is an illicit principle, and it is explicitly contrary to the principles of the Catholic Faith, according to which we must receive with docility the teaching of the Church, which is the Body of Christ.