Question: "What is the biblical doctrine of illumination?"
Simply put, illumination in the spiritual sense is “turning on the light” of understanding in some area. Throughout the ages, people in every culture and religion have claimed some kind of revelation or enlightenment from God (whether true or not). When that enlightenment deals with new knowledge or future things, we call it prophecy. When that enlightenment deals with understanding and applying knowledge already given, we call it illumination. Regarding illumination of the latter type, the question arises, “How does God do it?”
The most basic level of enlightenment is the knowledge of sin, and without that knowledge, everything else is pointless. Psalm 18:28 says, “You, O LORD, keep my lamp burning; my God turns my darkness into light.” Psalm 119, which is the longest chapter in the Bible, is a song about God's Word. In verse 130, it says “The unfolding of your words gives light; it gives understanding to the simple.” This verse establishes the basic method of God's illumination. When God's Word enters the heart of a person, it gives light and understanding to them. For this reason, we are repeatedly told to study the Word of God. Psalm 119:11 says “I have hidden your word in my heart that I might not sin against you.” Verses 98 and 99 say “Your commands make me wiser than my enemies, for they are ever with me. I have more insight than all my teachers, for I meditate on your statutes.”
Regular study of the Word of God will give direction and understanding in the issues of life. This is the first method of God's illumination and the starting point for us all. In Psalm 119 we also find another type of God's illumination. Verse 18 says, “Open my eyes that I may see wonderful things in your law.” These are not new revelations, but things which have been written and revealed long before, and just now understood by the reader (one of those “aha!” moments). Similarly, verse 73 says, “Your hands made me and formed me; give me understanding to learn your commands.” The plea is for personal understanding and application of God's laws as they are studied by the individual. Fifteen times in this psalm, God is asked to teach or give understanding regarding His laws.
One passage that sometimes stirs controversy regarding illumination is John 14:26, “But the Counselor, the Holy Spirit, whom the Father will send in my name, will teach you all things and will remind you of everything I have said to you.” Jesus was speaking to His disciples in the upper room, giving them last instructions before His death. This special group of men was to be responsible for spreading the good news of Jesus Christ to the whole world. They had spent three and a half years with Him, watching His miracles and hearing His teachings. They would relay those things to the rest of the world, and would need God's special help remembering those things accurately. Jesus said that the Holy Spirit would teach them and remind them of what had been said, so they could give it to others (including the writing of the Gospels). This verse does not teach that the Spirit will do so with all believers (though there are other verses that speak of the Spirit's illuminating work).
What is the Holy Spirit's illuminating work in believers? Ephesians 1:17-18 tells us that the Spirit gives wisdom and revelation concerning Jesus Christ, and opens the eyes of understanding so we can know God's purposes in our lives. In 1 Corinthians 2:10-13, God has revealed His plans for us by His Spirit, who teaches us spiritual things. The context here points to the Word of God as that which has been revealed. The Spirit of God will always point us to the Word of God for our instruction. As Jesus told His disciples in John 16:12-15, the Spirit simply repeats what the Father and the Son have already said. This repetition helps us remember and fully hear what God has already told us. Sometimes we have to hear things several times before we actually hear them. That's where the Spirit comes in.
One thing that is sometimes overlooked in the discussion of illumination is the purpose of it. To hear some arguments, it would seem that the whole purpose of illumination is an accurate and academic understanding of God's Word. There is no question that God desires us to accurately understand what He has given us. Words have meaning, and we must pay attention to the details in those words. If, however, we stop there, we simply have an academic understanding of facts or philosophies, which do no one any good.
Going back to Psalm 119, we find purpose statements connected with the illumination verses. “I will meditate on your wonders” (v. 27), “I will keep your law and obey it with all my heart” (v. 34), “that I may understand your statutes” (v. 125), “that I may live” (v. 144). The illumination always points to action. Why does God help us understand His Word? So we are able to live in its light. First John 1:6 challenges us, “If we claim to have fellowship with him yet walk in the darkness, we lie and do not live by the truth.” We could paraphrase it to say, “If we say we've been enlightened, but still walk in the dark, we lie about understanding God's Word.” The Spirit of God, who enlightens us to hear and understand God's Word, then takes that knowledge and guides us in living it. Romans 8:14 says “For as many as are led by the Spirit of God, they are the sons of God.” The illuminating and leading work of the Holy Spirit in our lives is a confirmation that we are indeed children of God.