There are a number of summer rituals that seem to take place every year. Being surprised that Wimbledon has come around so quickly, remembering that August's weather is about as good as November's and so on...

Another ritual is the denouncement of the Greenbelt Festival by British conservative Christians.

In 2009, there was outrage over the invitation of Gene Robinson, the first openly gay bishop of the Anglican Communion. In 2010 there was a call for a boycott over the invitation of gay human rights activist Peter Tatchell. Before and since there have been regular calls from conservatives to avoid the festival.

Greenbelt began in the early '70s as an evangelical event and has, of course, significantly broadened its horizons since then. At Greenbelt these days, you will find Christians of all denominations. Yes, there are some atheists too, among the crowds and on the stages. There are also Muslims, Jews and those of other faiths.

This year's controversy centres around the news that there will be a form of Muslim chant showcased in one venue. As Christian Today reported, the session will involve, 'teaching 'basic universal Sufi chants' which 'are rhythmic, healing and a unique form of worship' according to the Greenbelt website. Festival-goers are invited to 'come, enter in, learn and participate.''

This has prompted more discussion on social media, with one Christian media outlet sharing the news, prompting hundreds of angry comments from users.

I'm not going to get too bogged down in responding to this particular issue. Suffice it to say this is one session among hundreds, which is open to anyone, but in no sense integral to the programme. Muslims have been attending and speaking at Greenbelt for years and the sky hasn't fallen in. Those angrily commentating on this news don't seem to be people who've actually experienced Greenbelt and I'd be surprised if they knew the difference between a Sufi and a Salafi approach to Islam.

Personally, I don't believe Christians and Muslims should worship together, as we're fundamentally approaching God in a different way. In that sense, I've very much evangelical – the cross, scripture, personal conversion and activism are all important to my understanding of my Christian faith.

In spite of (or even because of?) that, I'm keen to find out more about the Muslim faith and to do so in a constructive environment in which we can all be respected and listened to. Greenbelt is one such environment. I'm also keen to interact with more Muslims. In a world where there are over a billion of them, and counting, that just seems like a common sense point of view.

Why, then, do I continue not just to attend Greenbelt but to volunteer there?

There are three reasons.

Firstly, there is loads of content that is 'evangelical'. A few years ago I recall vast queues to hear NT Wright speak. In a venue which held over 3,000 people, there were scores who couldn't get in, so popular was the New Testament scholar. Other evangelical voices I've enjoyed in recent years include theologian Alister McGrath, Archbishop Justin Welby, politician Baroness Berridge, activist Shane Claiborne and many more besides. I even held my hands aloft and sang myself hoarse (along with a packed Big Top) while Graham Kendrick led worship.

Secondly, what is the point in an event that takes place in a hermetically sealed environment? Where is the opportunity for growth if we are never challenged? I've heard stuff at Greenbelt that I disagree with. Heck, I've introduced acts at Greenbelt and then disagreed with them while standing at the side of the stage.

I don't see this as a problem to be covered up. It's a strength of the festival. There are plenty of other events which have their own theological line and stick to it – and that's OK. Greenbelt isn't like that, it challenges as much as it comforts. By its own admission, the festival doesn't always get it right, but without risk, there's less chance of reward.

Thirdly, I remain involved in Greenbelt because it is scratching in areas that other festivals don't itch. Touching on the urgently pressing issues of racial injustice, political disenfranchisement and the existentially important issues surrounding environmental destruction, the festival is an important prophetic voice. Other festivals are brilliant for expositional Bible teaching, extended musical worship and so on. But without that prophetic edge that Greenbelt offers, the whole body in the UK would be suffering.

Again, I don't agree with everything that happens at Greenbelt. That's kind of the point.

I've noticed a worrying trend among some Christian friends. Their propensity to judge Greenbelt without having even been is frightening. Yes, a gay bishop once took part, but does that mean the whole event is worthless? Some are willing to write off the whole event simply because they don't agree with the position of one speaker. In a world pulling apart over political, theological and philosophical differences, let's be careful before we criticise others. Perhaps the answer is even to get a ticket, come along and join the conversation?

Follow Andy Walton on Twitter @waltonandy