There's a little in-joke in the evangelical subculture around people who share the gospel for a living. The joke (and I promise you, it really is a small one) is that evangelistic preachers are prone to evangelasticity; which is to say that when they report back from their latest event or altar call, they somewhat increase the scale of what really happened.

A handful of responses becomes an outpouring of response. Two people becoming Christians turns into 'many were saved'. Tens become hundreds; hundreds become thousands. With a willing audience desperate to hear about revival-signalling swathes of new converts, it's just so tempting and easy to add on a zero, or to tell the story in such a way as it appears that a remarkable number of people have come to faith.

Evangelists exaggerate for a number of reasons, and not all of them are nefarious. Yes, they want to appear like they're good at their job – a need that's familiar to most of us – and yes, in some cases they're trying to live up to a heavily-funded narrative which places them at the centre of international revival. Often though, I think they're just trying to encourage the faithful, and the number that slips out just somehow gets doubled, or tripled, or...

Not all evangelists and preachers do this of course. Some are admirably committed to telling the truth, even if it makes them look comparatively ungifted. But this practice – so often laughed off as some sort of roguish foible – is much more common than it should be, and also extends beyond the field of evangelism, to event attendance numbers, book and resource sales, and of course, congregation size.

We lie about these things out of insecurity of course, a feeling that the truth just isn't impressive enough. So many of us lack confidence in the power of a single changed heart (especially when the event at which it changed cost thousands of pounds), or in God's ability to work extraordinarily in the half-filled room. But here's the truth: this scale issue is ours, not God's. In the parable of the Lost Sheep in Luke 15, Jesus says that 'there will be more rejoicing in heaven over one sinner who repents than over 99 righteous persons'. God is desperately and intimately interested in the one or two who truly respond to him; he throws a banquet in heaven for a single individual.

So evangelists: let's stop feeling the need to exaggerate. If you preach the sermon of your life, at an event that cost a fortune to put on, and a handful of people decide to follow Jesus as a result, than don't see it as a failure but a massive success. It's God who changes hearts anyway – not you – and he's delighted that three or four people, rather than the 30 or 40 you're inclined to report, have turned towards him.

To the rest of us: 'evangelasticity' isn't just something to laugh off, or indeed to embolden. Christians are people of the Truth, called to live with honesty and integrity, even when it comes to the numerical impact of our ministries. Of course we all long to see people to know and respond to the love of God in their thousands and tens of thousands, but until they do, let's not lie about it, or obsess so much about the idea that it encourages exaggeration in others. God rejoices when a single lost sheep is found; that should be enough – and incredible news – for us too.

Martin Saunders is a contributing editor for Christian Today and the deputy CEO of Youthscape. Follow him on Twitter @martinsaunders.