"The man without the Spirit does not accept the things that come from the Spirit of God, for they are foolishness to him, and he cannot understand them, because they are spiritually discerned." - 1 Corinthians 2:14
The knowledge of divine things to which Christians are called is more than a formal acquaintance with biblical words and Christian ideas. It is a realizing of the reality and relevance of those activities of the triune God to which Scripture testifies. Such awareness is natural to none, familiar with Christian ideas though they may be (like “the man without the Spirit” in 1 Cor. 2:14 who cannot receive what Christians tell him, or the blind leaders of the blind of whom Jesus speaks so caustically in Matt. 15:14, or like Paul himself before Christ met him on the Damascus road). Only the Holy Spirit, searcher of the deep things of God (1 Cor. 2:10), can bring about this realization in our sin-darkened minds and hearts. That is why it is called “spiritual understanding” (spiritual means “Spirit-given,” Col. 1:9; cf. Luke 24:25; 1 John 5:20). Those who, along with sound verbal instruction, “have an anointing from the Holy One... know the truth” (1 John 2:20).
The work of the Spirit in imparting this knowledge is called “illumination,” or enlightening. It is not a giving of new revelation, but a work within us that enables us to grasp and to love the revelation that is there before us in the biblical text as heard and read, and as explained by teachers and writers. Sin in our mental and moral system clouds our minds and wills so that we miss and resist the force of Scripture. God seems to us remote to the point of unreality, and in the face of God’s truth we are dull and apathetic. The Spirit, however, opens and unveils our minds and attunes our hearts so that we understand (Eph. 1:17-18; 3:18-19; 2 Cor. 3:14-16; 4:6). As by inspiration he provided Scripture truth for us, so now by illumination he interprets it to us. Illumination is thus the applying of God’s revealed truth to our hearts, so that we grasp as reality for ourselves what the sacred text sets forth.
Illumination, which is a lifelong ministry of the Holy Spirit to Christians, starts before conversion with a growing grasp of the truth about Jesus and a growing sense of being measured and exposed by it. Jesus said that the Spirit would “convict the world” of the sin of not believing in him, of the fact that he was in the right with God the Father (as his welcome back to heaven proved), and of the reality of judgment both here and hereafter (John 16:8-11). This threefold conviction is still God’s means of making sin repulsive and Christ adorable in the eyes of persons who previously loved sin and cared nothing for the divine Savior.
The way to benefit fully from the Spirit’s ministry of illumination is by serious Bible study, serious prayer, and serious response in obedience to whatever truths one has been shown already. This corresponds to Luther’s dictum that three things make a theologian: oratio (prayer), meditatio (thinking in God’s presence about the text), and tentatio (trial, the struggle for biblical fidelity in the face of pressure to disregard what Scripture says).