Jesus said: "If your hand causes you to stumble, cut it off; it is better for you to enter life maimed than to have two hands and to go to hell, to the unquenchable fire." (Mark 9:43).

I once heard of a person with a medical condition which meant they had developed gangrene in one of their legs.

The opinion of the specialists was clear: the limb needed to be removed, otherwise the infection would spread and prove fatal. The individual refused. They lived longer than the medics had anticipated, but eventually the prediction proved correct and the patient died.

That's very much akin to the picture Jesus uses in Mark 9, as we continue our fortnightly journey through this Gospel. If your hand causes you to stumble – or sin, as some translations put it – then we are to get rid of it, he says, because complacency will lead to fatality.

So serious is Jesus about this that he reiterates it in virtually identical words in the two verses that follow, speaking of cutting off a foot and then of even "tearing out" an eye. It's a brutal illustration.

It's universally agreed that Jesus is speaking pictorially here, rather than literally. He doesn't mean we should physically amputate our limbs or other organs. The early church father Origen of Alexandria (c185-254AD) was rumoured to have castrated himself, but it seems most likely this was malicious gossip put about by an opponent.

We need to think practically about what Jesus' teaching means, for example in relation to pornography and other similar issues. But before that, let's note that:

1. Something in itself good can become unhelpful, depending on how it is used. Hands, feet, eyes – these are good things, aren't they? They are not "sinful" in and of themselves. But it does depend on what we do with them!

2. Admission to God's kingdom may be free, but the annual subscription is everything.Or, to use more theological terms: salvation is free, but sanctification is costly. Entry into God's Kingdom is open to all because of what Jesus has done on the cross; getting fit for God's Kingdom means we can't just spiritually lounge around.

3. Jesus' warnings about hell are among the bluntest teachings in the whole Bible. Some people see talk of "unquenchable fire" as a picture – but as someone once put it, "I sure wouldn't want to be wherever it is that is pictured!" Hell is not a stick to beat us over the head with – it is a reminder that God is a God of Justice and that justice will one day be done. Of course we like this when it applies to Hitler or ISIS – but we tend to shy away from the fact that God's justice applies to us, too.

So how do we work out Jesus' teaching in practice?

1. Sex. Sex is great, and a gift from God. But when we seek gratification in a way which damages ourselves, a spouse or exploited individuals via pornography we will ultimately burn ourselves. A simple app such as Ever Accountable can help keep ourselves and our children answerable.

2. Money. Money is also good! But when it becomes our master, rather than a servant, it can choke our spiritual health. What about trying to become more generous, and giving more money away each month?

3. Speaking. This is hard. There is no app to guard our tongue! Prayer, practice, confession and repentance in a repeated cycle are sometimes the only way forward, as I have found in my battle to speak kindly to call centre salespeople who ring out of the blue at inconvenient moments.

Ultimately we are led once again to the foot of the cross where Jesus' very life was cut off for us and our sin... And we remember the response his love demands – as the hymn When I Survey puts it: "My soul, my life, my all".

The Rough Guide to Discipleship is a fortnightly devotional series. David Baker is a former daily newspaper journalist now working as an Anglican minister in Sussex, England.