What does Mark 9:45 mean?

Jesus continues His admonition to take concrete steps to avoid sin. He starts by saying it is better to be drowned than to discourage a little one from following Him (Mark 9:42) and that it's better to chop off your hands than to use them to do wrong (Mark 9:43).

Although Jesus' theological point refers to the literal, eternal hell, He is using the Valley of Hinnom as a visual representation. The term translated "hell" in English here is from the phrase tēn geennan. Also known as "Gehenna," this was the valley outside of Jerusalem which had hosted altars where parents burned their children in sacrifice to the pagan god Molech (2 Chronicles 28:3; 33:6). King Josiah destroyed the altars and turned the area into a rubbish heap where animal remains were burned (2 Kings 23:10). After the time of Malachi, during the four hundred years of silence, Jews started to identify Gehenna with hell.

This round-a-bout way of speaking about sin was common in New Testament times. By speaking vaguely about the act and concrete about the means, Jesus is telling His disciples to take concrete measures to avoid all kinds of sin. While, ultimately, it would be better to go without feet than sin with them, Jesus isn't endorsing self-mutilation. He's saying, for instance, don't allow your heart to tell your feet to walk you into sin.

Even so, the hyperbole and the symbolism Jesus uses show that there is nothing we can do to prevent us from deserving hell. Only His sacrifice can save us from sin.