Today sees the beginning of Thy Kingdom Come, the third annual prayer event launched as an initiative of the archbishops of Canterbury and York. It's been astonishingly successful as a way of uniting Christians from all denominations and traditions in prayer. But prayer, after all, is what Christians do, and we don't need much encouragement to do it.

What's different about this enterprise, between Ascension and Pentecost, is that it's focused specifically on prayer for conversions. Yes: Anglicans, Methodists, Baptists, Catholics, Pentecostals, New Churches and all shades in between, even those traditionally lukewarm about evangelism, are praying for more people to become Christians.

Some, no doubt, are motivated by the dread of approaching extinction. Some denominations, and some individual congregations, are not doing well. They see the time coming when they'll have to close their doors for good, and they are afraid. That might not be the best motive, but let's be honest.

A far better motive is to think, 'We have something that's worth sharing with the world. Let's pray that others will find it through us.' But the faith has been bruised and battered in recent years. It's been under intellectual assault from the New Atheists (they're a bit quieter nowadays, actually); it's lost credibility because of horrific child abuse scandals; it's competing in a markeplace of activities in which Sunday, for all too many people, is just another day. For many Christians, there's a crisis of confidence: do we still have a message that we're comfortable sharing? And Thy Kingdom Come is, at least in part, about pushing back against that and saying, 'Yes' to God: aligning ourselves with his purposes for the world and becoming the answers to our own prayers.

Here are three reasons why we should pray with confidence for conversions.

1. Because Jesus is true

There's a poem by John Betjeman which tends to appear at Christmas. It paints a picture of an idyllic traditional festive season, complete with holly, tinsel, family gatherings and gifts – 'Bath salts and inexpensive scent/And hideous tie so kindly meant'. But it concludes with the stunning statement that nothing at all 'Can with this single Truth compare –/ That God was man in Palestine/ And lives today in bread and wine.'

If we Christians are sensible, we'll be respectful of other faiths. All of the major religions contain deep wisdom. Properly practised, all of them make people live better. The fact that we have our own faith doesn't mean that we have to be dismissive of or antagonistic toward things that other people believe.

But Christians believe something remarkable: that 'God was man' in Palestine. That, in Betjeman's words, 'The Maker of the stars and sea' became 'a Child on earth for me'.

As John says in his Gospel: 'The Word became flesh and made his dwelling among us. We have seen his glory, the glory of the one and only, who came from the Father, full of grace and truth' (1:14).

The astonishing claim made by the Christian Church is that in Christ, God himself became visible and tangible. He contained himself in a human being. As Paul says, 'in Christ all the fullness of the Deity lives in bodily form' (Colossians 1:9). When we see Christ, we see God. What we know of God is interpreted and defined by Christ.

As truth claims go, they don't come any bigger than that. And the task of the Church is to say, 'If you are serious about wanting to know how to live well, and if you want to be sure that your life has an eternal significance, this is where to come.'

2. Because church is good for us

We don't always think so. Famously, CS Lewis in his Screwtape Letters has his senior devil advising his young acolyte Wormwood that if he really wants to put his 'patient' off church he should get him to notice the neighbours he generally avoids: 'Provided that any of those neighbours sing out of tune or have boots that squeak, or double chins, or odd clothes, the patient will quite easily believe that their religion must therefore be somehow ridiculous.'

But church is more than that.

Church in its essence is a new community of people who have a special relationship with Christ: who are 'in' him, as Paul repeatedly says.

It's a community where we learn a completely different way of life. We learn that to be really happy and complete, we have to turn away from striving to fulfil our own natural desires. Instead, we let ourselves be shaped by loving, attentive relationships to others, all in the context of loving attentiveness to God. And astonishingly, we find that instead of Christianity narrowing our horizons, stifling our individuality and forcing us to a dead conformity, it liberates us. In the slow, years-long process of discipleship, we're moulded into better people: kinder, more understanding, more forgiving and more generous – and more Christ-like.

3. Because Jesus people make a difference

A few years ago, in 2009, there was an advertising blitz by atheist campaigners. Comedian Ariane Sherine had the idea in response to Christian advertisements on London buses. With support from the British Humanist Association and Richard Dawkins, they wanted to raise £5,500 to put slogans on 30 buses in London for a month. In the end they raised more than £150,000 and the campaign ran all over the UK.

The slogan was: 'There's probably no god. Now stop worrying and enjoy your life.' I'm not sure it converted many people to atheism. But I thought it was self-indulgent and unambitious. The best slogan they could come up with in the absence of God was 'enjoy your life' and this in a world full of hunger, disease, warfare and want.

'Stop worrying and enjoy your life'?

Actually, most Christians do. But we're driven, too, by a knowledge that the world as it is isn't the world that God meant it to be.

Jesus didn't leave the poor unfed or the sick unhealed. His mission was to body, mind and spirit. Believing in God doesn't make us worry. It doesn't stop us enjoying life. But it means we cannot shut our eyes to the needs of the world.

Maximilian Kolbe, the Polish priest who died in Auschwitz after volunteering to take the place of a man sentenced to be starved to death, once wrote: 'The most deadly poison of our time is indifference.'

I believe we should be evangelical about calling people out of their indifference and into Christ's service.

'Thy Kingdom Come' is the most powerful prayer imaginable, because it encompasses so much else. It is a rallying-cry, a call to join God's own mission. Evangelism has nothing to do with self-preservation – it's an offer of life to the world.

For more information about Thy Kingdom Come, click here.

Mark Woods is the author of Does the Bible really say that? Challenging our assumptions in the light of Scripture (Lion, £8.99). Follow him on Twitter: @RevMarkWoods