John Gill Commentary Luke 21:18

"And not a hair of your head shall perish." — Luke 21:18 (ASV)

But there shall not art hair of your head perish .
] That is, without the will of God, as in (Matthew 10:29Matthew 10:30) or not one shall perish, but what shall be restored again: or the sense is, that though they should be betrayed by their friends, and hated, and persecuted, and imprisoned by their enemies, yet they should be no losers in the main; all things should work together for their good; and though even they should be put to death, yet that would be to their advantage, since instead of a temporal, troublesome life, they should enjoy an eternal and happy one: for this cannot be understood of entire preservation from all corporeal damages and hurt; seeing it is, before declared, that they should be put into prisons, and some of them put to death; nor of their preservation at the destruction of Jerusalem, for none of them was living at that time, but the Apostle John, and he was not in those parts.