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 truth; for He will not speak of His own initiative, but whatever He hears, He will speak; and He will disclose to you what is to come. He will glorify Me, for He will take of Mine and will disclose it to you. All things that the Father has are Mine; therefore I said that He takes of Mine and will disclose it to you. John 16:12-15, NASB.
Our mind is limited. We can absorb only so much. And when it overflows our abilities we end up bewildered and confused. That is the way it was with the disciples--they had reached the saturation point. But conditions change. And with new challenges and new situations comes the need for additional knowledge to both meet that new environment and to successfully navigate through it.
So it was with the disciples. As a result, Jesus told them that He had many things to tell them that they could not bear at that time. Some of them He would explain to them during the 40 days He spent with them between His resurrection and ascension. The Resurrection had changed everything for them. It had transformed their perspective on both Messiahship and discipleship. And Jesus used those days of instruction to advance them from being disciples to being apostles.
But the life of the church and the history of the world is a shifting canvas. As a result, Jesus sends the Spirit to continue to guide the church into all truth. The most significant step in that leading would be the inspiration of the books of the New Testament that put in permanent form not only the life and teachings of Jesus but the struggles and Spirit-inspired insights and experiences of the early church.
The prophecies of Revelation are a quite specific example of what Jesus had yet to tell His followers but knew they were not ready to hear during His earthly life. It is the Revelation from and about Jesus (meditated through the Holy Spirit). Repeatedly John declares to his readers, "Let him hear what the Spirit saith unto the churches" (Rev. 2:7).
The Spirit's guidance on how to apply and understand the Christ event didn't stop with the death of the last apostle. Paul tells us that the Spirit's inspired leading into all truth through the prophetic gift will last until the church reaches full maturity (Eph. 4:8-13). Thus the Spirit's role will last as long as we are on earth.
The most important thing we can do right now is to rededicate our hearts, minds, ears, and eyes to a clear hearing of the Word of Jesus through the Holy Spirit.