"When the Advocate comes whom I will send you from the Father, the Spirit of truth that proceeds from the Father, he will testify to me. And you also testify, because you have been with me from the beginning. "I have told you this so that you may not fall away. They will expel you from the synagogues; in fact, the hour is coming when everyone who kills you will think he is offering worship to God. They will do this because they have not known either the Father or me. I have told you this so that when their hour comes you may remember that I told you. "I did not tell you this from the beginning, because I was with you. (Jn 15:26-16:4a)
Persecution has been part of the history of Israel. The first one was in Egypt. God redeemed them from Pharaoh. Then by Assyrians and then by Babylonians they were tortured and carried away as exiles. These persecutions were followed by that of the Greeks and then by Romans. So the history has been a chain of persecutions and redemptions. Prophets and other leaders were there to guide them along the way of God.
Christianity faces the same events. The early Church continuously faced persecutions. Faith in Jesus Christ kept the people together. For Christians the persecutions are also from the religions (if you want to say the mother religion). Persecution was considered to be a worship offered to God. Paul was doing that before his conversion.
The promise that Jesus offers his disciples is the presence of the advocate. The advocate will speak the truth and constantly be with them. The presence of the Father and that of Jesus are assured to them. Whatever be the persecution, the presence of God is the security and consolation.
