Father Martin Luther's subjective approach to supernatural truth, which he based on private interpretation of Scripture, led to the subjective approach to natural truth by Enlightenment philosophers, which they based on private interpretation of reason.
Pope Leo XIII connected Luther's subjective approach to revealed truth with the similar approach taken by rationalists to reasoned truth. In his 1893 encyclical Providentissimus Deus, the Holy Father wrote:
In earlier times, the contest was chiefly with those who, relying on private judgment and repudiating the divine traditions and teaching office of the Church, held the Scriptures to be the one source of revelation and the final appeal in matters of faith. Now, we have to meet the Rationalists, true children and inheritors of the older heretics, who, trusting in their turn to their own way of thinking, have rejected even the scraps and remnants of Christian belief, which had been handed down to them. They deny that there is any such thing as revelation or inspiration or Holy Scripture at all.
Luther's exaltation of faith in his creed — sola fide, faith alone — wound up destroying much of the Catholic Deposit of Faith. In like manner, the exaltation of reason alone by the rationalist philosophers of the Enlightenment age destroyed much philosophical truth and the process of reasoning itself. This subjective approach, especially to moral truth, has led to the justification of persecutions against the Catholic Church, evident in the bloody French Revolution.
As a defense against the sophistry of rationalism, Pope Leo XIII recommended the work of St. Thomas Aquinas that applied Aristotle's objective philosophy to Catholic doctrine. He said, "The best preparation will be a conscientious application to philosophy and theology under the guidance of St. Thomas of Aquinas and a thorough training therein."
Watch the panel discuss how the Protestant revolt destroyed faith and reason in The Download— Luther's Pernicious Influence.