The disciples had God in the flesh teaching them directly, but they largely misunderstood. However, that did not mean they blew their opportunity forever. Upon Christ’s ascension, He teaches them that the Holy Spirit would be sent to bring them understanding about the things they had heard and seen. And this would not be for their own personal benefit. Through the ministry of the Holy Spirit, the apostles would receive revelation that was necessary to understand the saving work of Christ, along with his commandments and then pass it down to the church for edification.

Consequently, Peter states that what the apostles have delivered to the church in Scripture is more certain than his own experience of being an eyewitness of Christ’s bright, shining glory on the mount of transfiguration—2 Pet. 1:19 So we have the prophetic word made more sure, to which you do well to pay attention as to a lamp shining in a dark place, until the day dawns and the morning star arises in your hearts. 20 But know this first of all, that no prophecy of Scripture is a matter of one’s own interpretation, 21 for no prophecy was ever made by an act of human will, but men moved by the Holy Spirit spoke from God. That text highlights of the authority of Scripture, whereas the next passage puts emphasis on the sufficiency of Scripture—2 Tim. 3:16 All Scripture is inspired by God and profitable for teaching, for reproof, for correction, for training in righteousness; 17 so that the man of God may be adequate [or thoroughly], equipped for every good work.
These passages should cause us to carefully consider how we think about the truth. The Holy Spirit gave the apostles all the truth needed about Christ and his ways. If the Spirit was sent to glorify Christ by what he disclosed to the disciples, do we really think he left out some crucial truth that we need? Or that God couldn’t superintend the transcription process to make sure the Scriptures were copied in such a way that we would receive exactly what he desired for us? So then why do we think we need more revelation?—We just do not want to exercise faith. There are many examples of that:

Maybe we’re waiting for some new discovery about human behavior that will explain our issues and solve our problems in a quicker and more comprehensive way.
Or maybe we think: “I need revelation about your sovereign will Lord, namely how this situation will turn out—then I can trust you.”
Or…”I need the secret formula of sanctification that will get me out of this pattern of sin.”
Or “If I could just get some sort of sign or emotional affirmation, then I could have certainty to make decisions.”
What is being taught in John 14 is that the Father sent the Spirit to the apostles to give them all the truth of Jesus he wanted them to know and then they wrote down all that truth so that you and I would have everything we need for spiritual life and godliness. Therefore we need nothing else; we need to look nowhere else, but to grow in the true knowledge of God that the Spirit of truth uses to protect and guide us. Jesus will pray to the Father in John 17—sanctify them in the truth, your word is truth. That occurs through the work of the Holy Spirit inside us. Let us then glorify the Lord by believing in what he has given, rather than charge him with being stingy with the truth.