Jesus’s promise is not that the Holy Spirit would help the disciples to understand truth they had already received. Rather, the Spirit would guide them into the truth—all of it—that Jesus wanted them to have but that they were not yet ready to bear. In other words, these verses are about receiving truth (new revelation) and not about understanding truth already given (illumination).
Jesus cited, used, and endorsed every section of the Old Testament, whether law, prophets, or writings. Consequently, the Old Testament stands as a unit with His stamp of approval upon it. To reject its authority is to assail the authority of Christ Himself.
The authors of the New Testament had a very high view of their own writings. They asserted the authority of what they wrote, comparing it to the authority of recognized biblical texts and of the Lord’s own words. They also endorsed each other’s writings. To accept apostolic authority is necessarily to accept the authority of the New Testament.
A question arises, however, and it is an important question. Did Jesus ever endorse the New Testament? Does it stand beside the Old Testament with His stamp of approval upon it?
To discover Jesus’s opinion of the New Testament will require a different kind of evidence than His explicit endorsement of the Old Testament. By the time Jesus was born, the most recent document from the Old Testament was several hundred years old, widely distributed, and well known. Yet not one book of the New Testament was written during the earthly life and ministry of Jesus. If Jesus endorsed the New Testament at all, then He had to do it before it was written. His words about the New Testament would have to take the form of foretelling a later event.
Such words can be found in Jesus’s discourse on the night before He died, which appears in John 13–17. This discourse is divided by the departure of Judas in John 13:31. After Judas had gone, Jesus addressed the eleven remaining apostles. Most of what He said was directed specifically to them. When Jesus meant to include other believers, He either used indefinite language, such as when He referred to “every branch in me” (15:2) or broadened His reference with some phrase such as “them also which shall believe on me through their word” (17:20). In this discourse, when Jesus used the plural “you,” He usually meant specifically, “you apostles.”
He certainly meant the apostles when He said, “I have yet many things to say to you” (16:12). Throughout His ministry Jesus had been revealing new truth to His disciples. Here, on the last night before the cross, He told them that He had more to say to them. This was an intimation that His revelation to the disciples remained incomplete.
The reason it was incomplete is because the disciples were “not yet able to bear it” (16:12). They lacked some capacity for bearing up under the weight of the truth that Jesus wanted to communicate to them.