According to Jesus, to entice, trap, or to lead a believer into sin is a very serious matter. The hanging of a millstone around a person's neck and throwing them into the sea was a Gentile form of execution. Jesus said such a horrifying death is to be preferred to leading a Christian into sin.
The contrast of "life" with "hell" indicates that Jesus was referring to eternal life. The Greek word gehenna refers to the Valley of Hinnom near Jerusalem, a garbage dump where fires burned continually. Jesus used gehenna to serve as a graphic symbol of eternal torment.
According to Jesus, Hell is real. According to Jesus, Hell is a place. According to Jesus, you don't want to go there.
According to Jesus, I should be about the business of doing whatever it takes not to go there...
Jason Cicero