John 16:12-13a: “I have many more things to say to you, but you cannot bear them now. But when He, the Spirit of truth, comes, He will guide you into all the truth; …” The Lord knew that the disciples were not ready at that point to bear all that He could teach them. This may have been due to their slowness to believe all that the prophets had spoken, especially the truths related to Messiah’s suffering (Luke 24:25-26). Jesus had repeatedly told the disciples that He was going to die and be raised from the dead, but they didn’t get it until after His resurrection (Luke 9:22, 44-45; 24:45-46). And there were other truths that they could not comprehend until the Holy Spirit came to dwell in them permanently. Here Jesus promises that the Spirit would guide them into all the truth.
“All the truth” does not mean “all the truth about science or math or world history.” It refers to all spiritual truth that the apostles and the church needs for growth in godliness. As Paul writes concerning the glorious things that God has prepared for those who love Him (1 Cor. 2:10-12):
For to us God revealed them through the Spirit; for the Spirit searches all things, even the depths of God. For who among men knows the thoughts of a man except the spirit of the man which is in him? Even so the thoughts of God no one knows except the Spirit of God. Now we have received, not the spirit of the world, but the Spirit who is from God, so that we may know the things freely given to us by God ….
As a loving Father, He knows how much His children can bear, and so He patiently teaches us what we need for the stage of growth that we are in. A wise father doesn’t teach nuclear physics to his five-year-old. He teaches him the A-B-C’s, simple arithmetic, how to read, and other basic truths. As he grows, you take him deeper. The Holy Spirit does that with us spiritually. A young believer needs the milk of the Word: to understand what salvation means, who God is, how to live by faith, how to read and study the Bible, how to pray, etc. Later, he can begin to digest some meat (1 Cor. 3:1-3; Heb. 5:11-14; 1 Pet. 2:2).
In Ephessians 3:8, it is a never-ending process. That’s how your study of God’s Word should be. The Holy Spirit is the divine guide, who takes you from room to room, revealing the riches of Christ to your soul. Sometimes, you’re on your twentieth trip through a book and you see something that you’d never seen before, so you stop and revel at the glory of God in Christ. At other times, you make a connection between one part of God’s Word and another part that lets you see afresh that this book is not a product of human genius, but rather the inspired word of the living God. But you’ll never get to a place in this lifetime or even in all eternity (Eph. 2:7) where you can say, “I know it all; there’s nothing more for me to learn from the Bible!” So keep reading your Bible over and over, asking the Holy Spirit to reveal more of Christ to your soul.