The first half of chapter sixteen continues Christ's teaching from the end of the previous chapter: the disciples would be persecuted, but Jesus would send the Holy Spirit from the Father to the world (see 15:18-27). Jesus warned His followers that they would be persecuted and killed by people who never knew the Father or Son (16:1-4). St. John Chrysostom explains that we can gain comfort from Christ's warning:
If we reflect (on the fact that we suffer "for virtue's sake and for His sake"), everything will be easier and more tolerable. Since one is even proud when suffering for those he loves, what kind of feeling will such a person have who suffers for the sake of God? For if Jesus, for our sake, calls that shameful thing, the cross, 'glory' (13:31), how much more should we think that way...we ought then, when about to endure anything unpleasant, to think not of the toils but of the crowns (see James 1:12; Revelation 2:10).
The disciples were upset to hear that Jesus would be leaving them, but He told them that His imminent departure was a good thing: it would result in the Holy Spirit coming down to them (16:7). The Holy Spirit would do two things. First, He would convict the world of three things (16:8-11): sin (because of those who do not believe in Christ); righteousness (because, as Blessed Theophylact puts it, "The Spirit will make it clear to them that I am righteous: that I, the blameless One, was unjustly killed by them"); and judgment (because Satan, "the ruler of this world," is judged).
Secondly, the Holy Spirit would "guide (Jesus' followers) into all truth," glorifying Christ and repeating the Lord's teachings to His people (16:13-15). This passage is a profound statement about the Holy Trinity: as St. Ambrose of Milan says, "(The Holy Spirit) speaks the truth, He breathes wisdom. He does not speak without the Father, for He is the Spirit of God. He does not hear from Himself, for all things are of God...Therefore what the Spirit says is the Son's, what the Son has given is the Father's. So neither the Son nor the Spirit speaks anything of Himself."
The chapter concludes with Christ prophesying His Crucifixion, Resurrection, and Ascension. The disciples were confused by Jesus' statement that, after a little while, He would return to the Father and no longer be seen by them (16:16-18). Jesus then told them that they would engage in a period of sorrow followed by rejoicing, much like a pregnant woman experiences pain while in labor but then is overjoyed by the birth of her child (16:20-22). Because Christ would return to the Father, the disciples would be able to ask the Father for things in Jesus' name—something they did not do while Jesus was with them—and the Father would grant their requests (16:23-24, 26-27). This is because the Father loves Christians, and Christians can experience this blessing because we are united to Christ (16:27).
"Finally," the disciples said, "you're speaking to us plainly (instead of in figurative language)" (see 16:25, 29-30)! Now they seem sure that Christ knows all things (16:30). Christ replied—as the Church Fathers point out—that the disciples would demonstrate the limits of their belief by abandoning Christ during His Passion (16:32). Christ's words would nonetheless later be a source of comfort: even though they—and we—may suffer tribulation, Christ's victory over the world is total (16:33).