Some of the teachers of the Law ... asked Jesus' disciples, "Why does he eat with tax collectors and sinners?" Jesus heard them and answered, "Healthy people don't need a doctor, but sick people do. I didn't come to invite good people to be my followers. I came to invite sinners." (Mark 2:16-17)

Wouldn't it be great if some godly, thoughtful Christian could just hang around with Stephen Fry for a bit and share something of their faith?

The good news is that this has already occurred. This week – in the wake of the comedian's now infamous outburst against God – I happened to stumble across a snippet of video in which adventurer, writer and TV presenter Bear Grylls spends time with the famous actor and does just that.

What's brilliant is that this is a Christian simply doing what Christians should do: spending time with those far from the religious establishment and who seem unlikely candidates for any kind of faith.

Which is exactly what Jesus did, of course. Here he is, in Mark chapter 2, eating at the house of a tax collector called Levi and scandalising members of the religious establishment by eating with "sinners". Taxmen in those days weren't respectable if slightly boring members of society, of course – they worked for Herod Antipas (a ruler of great cruelty), were backed by the occupying Roman forces, and generally extorted more tax than was required.

It's into this context that Jesus, walking along by Lake Galilee, wanders in, sees Levi and says to him, "Follow me." And Levi, we are told, gets up, follows him and then invites him home for dinner.

The religious leaders are shocked to the core: doesn't Jesus realise what kind of man this is? And what about all the rest of them as well – that crowd of hangers-on, ne'er-do-wells and riff-raff? As the murmurs about his behaviour reach his ears, Jesus responds – as The Message translation paraphrases it – with devastating simplicity: "Who needs a doctor: the healthy or the sick? I'm here inviting the sin-sick, not the spiritually-fit."

So what does this tell us?

1. Jesus calls the unlikeliest of people. Indeed, when he chose his disciples, he bypassed the obvious, spiritually-trained candidates and called together a wildly varied and unusual group. Never regard anyone as a lost cause for Christ. Whether it's a neighbour, a colleague or "even" a Middle East terrorist leader – we can keep on praying for them all.

2. God's grace can transform anyone. As one writer says: "God's grace rips up the merit system of human attainment and forgives, and eats with, and befriends rebels who find it impossible to work their way into God's good books." Jesus transformed Levi and he's done the same with all sorts ever since – from slave trader John Newton to gangster Nicky Cruz.

3. The world needs a doctor. It's ironic that one of the BBC's most popular programmes globally is about a man called The Doctor who goes round righting wrongs, helping humanity and enabling people to discover who they really are. Television's Doctor Who – as many have pointed out – is not only great entertainment but resonates with human longing on all sorts of levels. However, it was Jesus who first applied this title to himself in real life.

4. We have to recognise our sickness. Jesus is saying that he's a physician and he's come for those who are ill. In a way, it's quite a provocative – even insulting – picture. And that's the point, of course. If we reckon we are "righteous" – as the religious leaders did – and fail to recognise our sin and need of Jesus then his invitation and description will merely bemuse and/or offend us. Self-righteousness comes in many forms – whether religious, secular or liberal. All are self-blinding.

But when we see what Jesus is about, and understand his diagnosis, we'll be first in the queue for that dinner party with him. Let's pray that one day Stephen Fry is there celebrating too.

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.