We've all experienced it. There's someone who's been around for years, part of the church furniture. They're reliably present at Bible studies and turn up for working parties and lunches. But so gradually it's hardly noticeable, they appear less often. Evening service drops off, they disengage from church activities and over a few months they fall off the radar completely. 'What happened to so-and-so?' 'I'm not sure – he doesn't come any more.'

What's happened? There might be a number of reasons. Perhaps there's been an unacknowledged falling-out with another church member and it's festered. Perhaps life events have intervened – a family illness or change in working patterns might make church attendance difficult.

But perhaps there's something deeper going on, and we really don't like to talk about it. Evangelical Christians love to talk about conversion – it's what we do – but the flip side of that is virtually taboo. Because just as people find faith, sometimes they lose it. Call it de-conversion.

At one level this is, in some theological understandings, a problem in itself. A Calvinist who believes in the 'once saved always saved' mantra is reduced to arguing that someone who loses their faith either never had it in the first place, or that they haven't really lost it. In terms of someone's actual experience, though, that isn't what it looks like.

It isn't always possible to stop this 'faith drain'. It's a function of human free will, and the process of growth and change that all of us experience throughout our lives. It shouldn't surprise us – obsessions about football, politics and other human activities ebb and flow as the years go by, and faith, too, is variable. But at the same time we offer it as a life-enriching discipleship that is life-long. If someone finds it's not, we need to ask why – and learn to do it better.

Here are three things that might cause faith to wither.

1. It stops relating to life

When we read the Bible, we're often advised to ask, 'What does it say, what does it mean, and what does it mean to me?' That last question is key: we can be taught all sorts of fascinating Bible facts, but unless that leap is made to relevance for me, it won't matter. Discipleship becomes sterile, and we'll find our real interests and passions elsewhere. The hard intellectual work of relating the gospel to the 21st century has not been done. The risk then is that one day we wake up and think, 'What would be missing from my life if I weren't a Christian?' and don't have a good enough answer.

2. It becomes incredible

In the proper sense of 'unbelievable'. We need to be very, very careful what we claim for our faith. If we hitch it to particular causes or make claims not supported by evidence, we risk undermining our claims to absolute truth. As Dean Inge said, 'Whoever marries the spirit of this age will find himself a widower in the next.' Evangelicals in the US, who support Donald Trump in far greater proportions than the wider population, may find this to their cost. Christians adamant that the world is only around 10,000 years old will find themselves increasingly marginalised. We can't be isolated from wider society, and wise, thoughtful engagement rather than denial is the way to keep faith strong.

3. Church can over-promise and under-deliver

Some churches provide full-on, high-octane, all-embracing, immersive religious experiences. Worship services are inspirational emotional highs. Pastors preach brilliantly about what God can do. And it's great – until God doesn't. As preacher Christy Wimber said recently, 'My concern for the charismatic church is there is no door for the mentally ill to come in. There is no theology of suffering.' If our life experience doesn't match up to what our church promises – joy, healing, success, fellowship – it's easy to become disillusioned.

Pastors and elders won't always be able to make sure someone remains faithful to their baptismal promises or committed to the church community. That's how things are. But we can help, by listening to people's doubts and uncertainties, keeping windows open to the world so we don't become sealed off in a spiritual bubble, and keeping our fingers on the pulse of people's real life experience.

Follow Mark Woods on Twitter: @RevMarkWoods