Jesus said, "Your arrests will give you the opportunity to testify. You don't need to worry about what you are going to say because I will give you the words and wisdom that none of your opponents will be able to withstand or contradict."
They are not to take this as an assurance that nothing bad was going to happen to any of them. They are not exempt from suffering. Rather, some of them were going to be betrayed by relatives and friends, some of them were going to be put to death, they were going to face hatred because of their loyalty to him.
Yet, he added, "But not a hair of your head will perish. By your endurance you will gain your souls." Historically, tribulation did befall the early church. And, historically, God has continued to support the community through conflict--internal and external. Sharon Ringe in her commentary on Luke writes:
Even a false assurance (21:18)--for many would indeed suffer harm--echoes an earlier word of comfort (12:7). The final promise is not that they will be spared the suffering, but rather that their hope lies in endurance (21:19)--standing firm and refusing to give in to the evil around them.