At the outset of John 16, Jesus tells why he has told the disciples what he told them at the end of chapter 15. So let's turn a page back and remember what Jesus has just been telling the disciples . . . people will hate them and persecute them because they hated God first, but believers will be strengthened by the Holy Spirit, the Helper Jesus will send and who proceeds from the Father.
Now, why did Jesus tell the disciples all he told them? To strengthen them and keep them from falling away. Jesus here presents the very real possibility that true believers will face persecution, specifically physical and societal threats (being killed or cast out of the synagogue), and will depart from their faith in Jesus in order to protect their physical and social well-being. Jesus encourages his people to trust in him and his provision, not to fear those who can disrupt their earthly lives.
There are a few aspects of this brief paragraph which I think are very important. First, it is a very real possibility that genuine Christians may depart from the faith when facing persecution. I was speaking about this with two Calvinists a few days ago, both people whom I respect greatly, people with what appears to be a deep-seated trust in Christ and assurance of his love for them. As I understood the conversation I had with these friends, people who depart from the faith were not believers to begin with. While that point of view protects the Calvinist's high regard for the effectiveness of Christ's salvation - he will guard all genuine believers forever - it does not seem to fit in with what Jesus says here. Jesus, though he is certainly able to keep his people secure in him, allows people to flee from their trust in him and depart from the faith. He warns against it. He provides teaching about the ultimate safety of the believer. He provides the Holy Spirit to encourage and confirm believers. But believers appear quite able to depart from their faith.
A second aspect which is important to realize in reading the beginning of John 16 is that unbelievers, by and large, think they are doing the right thing by rejecting Christ and his people. They do not understand the way our Lord works salvation. They do not understand God's love in Christ the suffering servant. They don't understand the glory of laying down your own life, hopes, dreams, and future to receive that which Jesus has prepared for you. God's kingdom is counter-intuitive. We cannot expect that everyone will fear, love and trust God in Christ. This is one of the reasons I think a seeker-sensitive church growth model of ministry is dangerous. It approaches life and ministry with an attitude that says the world will understand Jesus, embrace him, and follow him if we present him in terms they are already predisposed to accept. Yet these terms run counter to the nature of God. Let God be God and every man a liar!
The final aspect of this passage which I'd like to mention is, in my opinion, the most important. However, I'll treat it in the briefest of terms because it is readily understandable. God in Christ has seen our future. He knows our dangers. He knows the struggles we will face. He has acted in time and space to protect us and carry us through those struggles. When we face discouragement and doubt in our lives, or if we face genuine threats to our life and well being, we can know that our Lord understands and has provided his grace for us. He has spoken in advance to protect us from the trials to come. Let us rejoice that our Lord knows our frailty and has anticipated all that will come upon us. Let us look to him in his resurrection, trusting that he has indeed overcome the world. By faith, we will overcome the world as well.