What's your biggest problem?

In fact, let's broaden out that question and re-phrase it: "What's the biggest problem we face today?"

A friend who is a church minister was once asked that question by a market researcher who arrived unexpectedly on his doorstep.

What, she inquired of him, was the greatest issue facing the world? Was it things like global warming, or the economy, or terrorism? My friend paused and then replied simply: "The biggest problem we face is sin."

I don't think his response fitted any of the neat categories the canvasser had on her list! But he was right, of course. According to Jesus, the biggest problem you have – and I have, too – is sin.

And when we talk of sin we're not talking about "doing naughty things" like over-indulging in cream cakes. We're not even talking solely about breaking commandments by stealing, coveting, committing adultery or failing to love God with all of our being.

In biblical terms, sin is rebellion against God – leading a life which (like the word "sin" itself) has "I" in the middle, rather than being centred on Him. Every single way we fall short and fail are symptoms of that off-centre kilter of our hearts.

It's for that reason that Jesus declares: "Listen to me, all of you, and understand: there is nothing outside a person that by going in can defile, but the things that come out are what defile," (Mark 7:14-15).

The context of his preaching is the external religious regulations about things like food and washing which many religious people of the time thought would somehow make them right with God. But they won't, says Jesus. Just as today's equivalents (being baptised, taking communion, reading the Bible etc), while all being good in and of themselves, won't deal with the main problem we have – the sin in our hearts.

That's why Jesus goes on to say: "Do you not see that whatever goes into a person from outside cannot defile, since it enters not the heart but the stomach, and goes out into the sewer... For it is from within, from the human heart, that evil intentions come: fornication, theft, murder, adultery, avarice, wickedness, deceit, licentiousness, envy, slander, pride, folly. All these things come from within, and they defile a person."

Someone has summed this up by saying: the heart of the problem is the problem of the heart. It's from our hearts that sin and its poisoned fruits spring.

A few days ago I saw part of the film The Voyage of the Dawn Treader based on the CS Lewis novel. One character echoes what Jesus is saying here, as he tells those setting off to confront various dark forces: "To defeat the evil outside, you must first confront the evil in yourselves."

So politicians can offer all sorts of economic solutions to the world's problems – but whether laissez-faire, or more interventionist, if they ignore the problem of the human heart they will always be fatally flawed. Similarly, we can come up with all sorts of ideas for countering the so-called Islamic State – but while we might eradicate the physical presence of terrorist rule, only changed hearts will bring transformed worldviews.

At this point, Jesus doesn't tell us what his solution for our hearts is. He simply leaves us the challenge. As Psalm 139:23-24 puts it: "Search me, O God, and know my heart; test me and know my thoughts. See if there is any wicked way in me, and lead me in the way everlasting."

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.