Lk 21:12 People should remember that before the gospel was written down, it was preached by the apostles for about 40 years in various parts of the Roman Empire. During this time, some persecution would have occured. Evidence for this would be found in the ministry of St. Paul, for example Acts 13:50. Thus the Lord's warning is reinforced by the community's contemporary experience which served to brace christians for the more serious persecution by the government that would begin under Nero.
Luke 21:13 God is able to bring something good out of a bad situation. He would not want His poeple to suffer but if evil people exercise their free will and choose to create suffering for His people, He will use the event to advance the truth. His people will be able to witness, to teach those listening. See Acts 23:1-11.
Luke 21:14-15. Because the person on trial is being called to do God's work, that is to witness to the truth, he can rely on God. The reliance is twofold. 1. God will give you what to say, and 2. God will protect you. This reliance on God to provide the defense is very different than what is expected in an American court. The modern trial is carefully scripted. What is done, and what will be said usually is known beforehand by the parties. The attornies do come into the situation "cold" but have carefully worked out how to handle the presentation of evidence. God, however, does not work like people work. The defendant, persecuted for faith, cannot control what will happen and so his only choice is to surrender to God in complete trust and to respond out of faith when questioned. Priests and preachers can testify that this surrender in trust works. Often enough when someone asks a question you have to trust God to prompt you with a good answer, or in preaching, even if you have worked out what you want to say, the Spirit of God may prompt you to use different material. In my personal experience, prayer is often the best preparation for preaching, teaching, or the unexpected encounter.
Luke 21:16-17 This must conform with the experience of the early church. As someone comes home with a new teaching the family will be upset. People are always expected to conform with family tradition and cultural expectations. In our time, there is somewhat less pressure because change is often built into the culteral expectations in the western way of life. However this is relatively recent and not universal. In prior times it could be dangerous to fail to conform. If a jewish family had a child become christian, the family might hold a funeral. A pagan family might cherish its beliefs just as much. The christian message moving through the ancient world would undoubtedly cause much conflict. Sometimes that conflict could spill out into the courts, especially in times of persecution, and result in death.
Luke 21:28-19 This promise seems unrealistic until one meditates on the phraise "gain life". The person would already have physical life, and even spiritual life if he or she had surrendered to Christ but the Lord through the evangelist promises more. What is gained? Everlasting life. Traditional belief held that a person who was martyred would automatically gain heaven*. Certainly, if someone witnesses to the faith in any difficult situation he or she confirms her election, allows God to be present in and through her for the listeners, and should suffering occur that suffering is joined to the suffering of Christ for the salvation of the world. Christ died and rose, and so shall we who join ourselves to Him.