Calvinist blogger, Tim Challies explains how illumination of the Holy Spirit is the distinguishing factor of being a Christian.
Illumination is what separates believers from unbelievers when we read the Bible. An unbeliever may read the Bible and view it merely as a religious or historical document, much like I would read the Koran or the Book of Mormon. But when a Christian reads the Bible, the Spirit guides him to see not merely history and religion, but the very words of God. And even more important, He allows the person to apply the great truths of the Bible to his life. He initiates change through the words of the Scripture. Being a Christian, then, is a necessary prerequisite for the Spirit’s illumination.
(“Revelation, Inspiration and Illumination by Tim Challies”, Credo Magazine October 2011 issue, pg. 66)