Jesus came into Galilee, proclaiming the gospel of God, and saying... "Repent, and believe in the good news." (Mark 1:15)

One of the most dramatic pictures of this year showed a lightning strike on London's tallest building, the Shard.

There's something about these sudden, illuminating and extraordinary powerful bolts of electricity that is unique. They jolt us away from whatever we are doing and immediately grab our attention.

And that's very much the impact Jesus intends as he starts his ministry. Not for him a quiet warm-up message with a few homespun jokes. No, Jesus cuts straight to the point: "The kingdom of God has come near; repent, and believe in the good news." As one writer, Jeremy McQuoid, puts it: "Jesus did not ease His way gently into Galilean hearts. He set off a shrill alarm to awaken a spiritually decadent culture."

But today there's a bit of a problem with repentance if we're honest. The word too easily brings to mind Christians at their worst – perhaps a hard-line, shouting Ulsterman or the notorious Westboro Baptist Church in Kansas.

In response to this, some have gone to the other extreme, apparently downplaying the idea altogether. One review of Steve Chalke's book The Lost Message of Jesus noted that "he rightly states that 'God's love is available to everyone' but does so without any mention of the fact that forgiveness is received by repentance and faith".

But Jesus seems quite clear on the necessity of repentance: in response to what many might consider a quite reasonable question about a building disaster, he declares, twice: "Unless you repent, you too will all perish," (Luke 13:3,5).

Most Christian leaders recognise this. Thus in 2013 Pope Francis declared that our basic problem as humans "is not repenting of sin, not being ashamed of what we have done". And veteran preacher Billy Graham states: "The Bible commands it, our wickedness demands it, justice requires it, Christ preached it and God expects it. The divine, unalterable edict is still valid: 'God commands all men everywhere to repent'."

What is repentance? Literally, it means "turning back". The invitation to repent is an invitation to return to the God in whose image we are formed and in whose presence life starts to make sense. It is an invitation to turn away from a self-centred life to a Jesus-centred life, to receive the gift of forgiveness and life which he offers, and to replace destructive patterns of behaviour with Christ-like habits and rhythms. It is the invitation to move from death to life.

Repentance is essential to kick-start the Christian life; it is also an ongoing daily necessity, and we never lose the need for it. Sometimes we need a fresh lightning bolt to startle us again. A while ago I was in a meeting away from the parish in a non-Christian context. The discussion was difficult and I spoke wrongly. Then I realised, to my horror, that one of the others present was a Christian – and she had recognised that I was too. My sinful hypocrisy was exposed. Later, she took me to one side and we sat at a table, sipping tea. Crestfallen and ashamed, I stuttered: "I'm sorry." She looked me in the eye, held my gaze for a moment, and declared: "I forgive you."

You see, repentance is not some abstract, theoretical exercise. It is the way relationships are restored – both with God and with other people.

The first Anglican Bishop of Liverpool, JC Ryle, wrote: "Let us ask ourselves what we know of this repentance and faith. Have we felt our sins and forsaken them? Have we laid hold on Christ and believed? May we never rest till we know them by experience and can call them our own."

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.