What were the “many things” in John 16:12?
We don’t really know exactly what Jesus had in mind. There may be a clue in the following verses when he says, “He will declare to you the things that are to come” (John 16:13). This may be apocalyptic and in times material like what John covered in Revelation.
While Jesus taught on the end times in the last days and what they would be like, much of his teaching is filled in throughout the epistles that were to come after him. The Holy Spirit revealed to the apostles all of the things they taught in these books.
Much of eschatology is fleshed out throughout epistles like 1-2 Thessalonians, 1 Corinthians, and Jude. The book of Revelation gives us a glimpse into Christ’s glory and how he will defeat the enemy in the end times.
The Holy Spirit led the apostles to give more and further teaching on all kinds of subjects that the church would deal with. Jesus covered the essentials but allowed the Holy Spirit to finish the essential teaching throughout the rest of the New Testament.
Other than this, Jesus may have finished his teaching after his resurrection and before his ascension. The Bible tells us that when he met with the disciples he opened their minds to the Scriptures and showed them how he fulfilled the Old Testament (Luke 24:44-49).
Through revealing himself completely to them as the promised Messiah of the Old Testament, Jesus probably finished the teaching that he could not share with them before he was crucified, died, was buried, and resurrected. What we have in the New Testament epistles probably contain much of his final teaching before he ascended to heaven.