From: John 15:26-16:4a
A Hostile World (Continuation)
(Jesus said to His disciples,)  "But when the Counsellor comes, whom I shall send to you from the Father, even the Spirit of truth, who proceeds from the Father, He will bear witness to Me;  and you also are witnesses, because you have been with Me from the beginning.
The Action of the Holy Spirit
 "I have said all this to you to keep you from falling away.  They will put you out of the synagogues; indeed, the hour is coming when whoever kills you will think he is offering service to God.  And they will do this because they have not known the Father, nor Me. [4a] But I have said these things to you, that when their hour comes you may remember that I told you of them."
26-27. Just before the Ascension our Lord will again charge the Apostles with the mission to bear witness to Him (cf. Acts 1:8). They have been witnesses to the public ministry, death and resurrection of Christ, which is a condition for elonging to the Apostolic College, as we see when Matthias is elected to take the place of Judas (cf. Acts 1:21-22). But the public preaching of the Twelve and the life of the Church will not start until the Holy Spirit comes.
Every Christian should be living witness to Jesus, and the Church as a whole is a permanent testimony to Him: "The mission of the Church is carried out by means of that activity through which, in obedience to Christ's command and moved by the grace and love of the Holy Spirit, the Church makes itself fully present to all men and peoples in order to lead them to the faith, freedom and peace of Christ by the example of its life and preaching, by the sacraments and other means of grace" (Vatican II, "Ad Gentes", 5).
2-3. Fanaticism can even bring a person to think that it is permissible to commit a crime in order to serve the cause of religion--as happened with those Jews who persecuted Jesus to the point of bringing about His death, and who later persecuted the Church. Paul of Tarsus was a typical example of misguided zeal (cf. Acts 22:3-16); but once Paul realized he was wrong he changed and became one of Christ's most fervent apostles. As Jesus predicted, the Church has often experienced this sort of fanatical, diabolical hatred. At other times this false zeal, though not so obvious, takes the form of systematic and unjust opposition to the things of God. "In the moments of struggle and opposition, when perhaps `the good' fill your way with obstacles, lift up your apostolic heart: listen to Jesus as He speaks of the grain of mustard seed and of the leaven. And say to Him: `edissere nobis parabolam': explain the parable to me.'
"And you will feel the joy of contemplating the victory to come: the birds of the air lodging in the branches of your apostolate, now only in its beginnings, and the whole of the meal leavened" ([St] J. Escriva, "The Way", 695).
In these cases, as our Lord also pointed out, those who persecute God's true servants think they are serving Him: they confuse God's interest with a deformed idea of religion.
4. Here Jesus prophesies not only His own death (cf. Matthew 16:21-23) but also the persecution His disciples will suffer. He forewarns them of the contradictions they will experience so that they will not be scandalized or depressed when they do arise; in fact, difficulties will give them an opportunity to demonstrate their faith.
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.