I say to you, my friends, Do not be afraid of those killing the body and after these things not having anything beyond to do. 5 But I will show you whom you should fear. Fear the one who after killing has authority to cast into the Gehenna. Yes, I say to you, fear this one.
This saying presupposes the threat to their lives that the disciples would face when they announced the coming of the reign of God to Israel (cf. Matt. 10:27). If they are killed by the Jewish authorities because of their witness, they have nothing more to fear because God will watch over them (12:6) and they will be vindicated when the Son of man receives the kingdom from the Ancient of Days (12:8-9).
What they should fear, however, is being caught up in the judgment that is coming on Jerusalem. They should fear the death and destruction that is the outworking of God’s wrath towards his people. They should fear being killed and then thrown into the valley of Gehenna. The valley of Hinnom in Jeremiah 7:30-33; 19:1-13 is the “Valley of Slaughter” where the people of Judah and Jerusalem will be slain by their enemies to become food for wild animals, or where they will be buried for lack of space elsewhere. Jesus is recalling very accurately Jeremiah’s image of the judgment on Jerusalem. So there is a death that will result in vindication, and there is a death (such as would be suffered by many during the war) that will result in the body being thrown into the valley of Gehenna as an outworking of God’s judgment against the city.