A gracEmail subscriber in The Netherlands (Holland) asks whether Luke 12:5 means that God will execute the wicked before casting them into hell. In this passage, Jesus says to fear God "who, after he has killed, has authority to cast into Gehenna."
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Gehenna, the Greek word translated as "hell" in the English Bible, is a name for the Valley of Hinnom just outside Jerusalem. In ancient times, this site served as the city's garbage dump, and it became a symbol for the final destiny of the lost. The idea you suggest certainly is in keeping with Isaiah 66, for example, which is the major background text for New Testament teaching concerning hell. Isaiah first says that the LORD will execute judgment by fire and by sword and that the slain of the LORD will be many (v. 15-16). The prophet then portrays the saved looking on the corpses in Gehenna which are being consumed by maggots and by fire (v. 24; see also Dan. 7:9-11).
Some Christians believe that Jesus will execute the lost in consuming fire at his final return, then cast their corpses into hell. Others, including myself, note scriptural language which suggests that God will banish the lost into hell where they will then be consumed. Matthew 10:28, the parallel passage to Luke 12:5, says that people should fear God "who is able to destroy both soul and body in hell." I do not think the language in Luke is setting out a chronological timeline. It is rather contrasting human power to kill (but then to do no more) with God's power which goes beyond death into the Age to Come.
When the Bible speaks of the Age to Come, it talks about realities that transcend the physics of this earthly Age and order. Biblical language concerning the Age to Come is analogical, metaphorical and symbolical. It tells us what those realities will be like -- but we cannot now fully understand what they will actually be. We ought to take such language seriously but we need not take it literally. What the Bible does make clear is that the time will come when evil and evildoers are gone forever and God's glory will fill the universe (Isa. 24:14-16; 2 Peter 3:13).
For more on final punishment, click here.