This protest was a response to the Catholic position that only those in communion with the Bishop of Rome can properly, authentically, and accurately interpret Scripture. This doctrine is explained in article 85 of the Catechism of the Catholic Church, and although modern Catholic thought does not exclude the possibility of someone landing on a proper conclusion apart from the Magisterium in communion with the Pope, Roman Catholic thought during the time of the Reformation functionally asserted that the Scriptures were not able to be interpreted by anyone apart from the Magisterium. In response to this the Reformers boldly asserted that the Scriptures are clear in and of themselves, and that no special Charism (spiritual empowerment) is required in order to properly interpret the Scriptures. The Westminster Confession asserts that although not all Scriptures are equally clear, that things that are required to acknowledge for salvation are clear enough that even the unlearned are able to understand them.
"The task of giving an authentic interpretation of the Word of God, whether in its written form or in the form of Tradition, has been entrusted to the living teaching office of the Church alone. Its authority in this matter is exercised in the name of Jesus Christ."47 This means that the task of interpretation has been entrusted to the bishops in communion with the successor of Peter, the Bishop of Rome. - Catechism of the Catholic Church 85
All things in Scripture are not alike plain in themselves, nor alike clear unto all (2 Pet. 3:16); yet those things which are necessary to be known, believed, and observed for salvation, are so clearly propounded, and opened in some place of Scripture or other, that not only the learned, but the unlearned, in a due use of the ordinary means, may attain unto a sufficient understanding of them (Ps. 119:105, 130). - Westminster Confession of Faith 1.7
I have pretty bad vision. If I am not wearing my glasses, I cannot see anything clearly that is more than about a foot and a half in front of my face. When I wear my glasses, the object or text in front of my face is able to be seen clearly to me.
Now, it is clear from this example that the problem causing a lack of clarity does not originate in the object, rather it originates in a defect within my eyes. This causes me to see something that is clear as though it were not.
This is where illumination comes into play. Just as we may see something incorrectly in the dark, that suddenly comes into sharp focus when the lights are turned on, we often need the Holy Spirit to bring light (not revelation) to a text in order for us to see something we previously did not. Perhaps it is something as simple as understanding on a deeper level what it means that God so loved the world that he gave his only Son, or a fuller perspective on what it means to be saved by grace through faith. Either way, just as putting on glasses or turning on a light helps us to see a clear thing more clearly, the illumination of the Holy Spirit enables us to see clearly that which is already clear by repairing our spiritual eyes and casting light to expel the shadows that may distract or obscure our vision.