Luke 21:18 relates with Matthew 10:30 and Luke 12:7, where Jesus teaches his disciples the hairs on their heads are all numbered. The context of those verses is Jesus teaching His disciples how much God cares for them, and yet to fear spiritual death more than physical death, because man can do no more than kill their physical bodies, whereas God is more than able to know what everyone is due for their actions, because He knows everything, and not even the sparrows die without God knowing. So, relating the context of those two verses back to Luke 21:18, Jesus is telling His disciples that even though they should suffer physical death through persecution, their spiritual bodies will be saved and they will lose no-thing of what they need to live with Him, forever, when the Son of Man enters His kingdom and the saints receive their inheritance in the kingdom of God - it will be as though they had not suffered any loss, if they hold firm to the end. The overall context of this verse, Luke 21:18, is Jesus showing His disciples that even though they are going to suffer because of their belonging to Him, and bearing His name, God is more than able to look after them in their sorrows, even to the point of persecution and death.