Pope Francis is under fire again, this time from Muslim converts to Catholicism who can't understand why he is, as they believe, soft on Islam.

A letter signed by former Muslims and their allies has attracted around 3,700 signatures. It accuses him of not understanding the danger posed by Islam and urges him to be more combative in his approach.

Catholicism is in many ways a world of its own, but this little ruckus is instructive for evangelical Christians too. In the evangelical world as well, there are those who are utterly opposed to the idea that Islam is anything other than the work of the devil. There's no point in conversation with Muslims; they're the enemy, and that's that.

But it's worth taking a moment to think about what's going on here. And the truth is that people who leave one religion for another – whichever religion it is – are going to have a pretty bleak view of the one they left. They may have been exposed to its worst, whether it's prejudice, fundamentalism or outright violence. For whatever reason, it doesn't satisfy their souls. They've found something else that does. The psychological adjustments needed for that are often immense. Some converts – by no means all – cope by demonising their former faith. If anyone doesn't join in their condemnation, they become aligned with the enemy.

And these are the narratives that get shared in books and on platforms, and that influence how a wider audience sees things. There's a market in Christian circles for testimonies of conversion, from Islam or Hinduism. That's understandable, and it's right that such stories should be told: those who tell them have sometimes suffered greatly, and they can provide an insight into a different culture we badly need to understand. Their upbringing and experiences have given them a certain authority to speak about another faith, and they have a right – even a duty – to highlight its negatives. 

But while they can be guides, they shouldn't be our only guides. Islam, for instance, is often regarded in the West with suspicion and fear because of the terrorist outrages perpetrated by Islamists. An ex-Muslim who's experienced that fundamentalist culture can reinforce that narrative by saying, 'Yes, that's what Islam is like.' But it's still only part of the story.

We don't learn about another faith by listening only to those who've been hurt or disillusioned by it. We learn by hearing those for whom it really works and by taking seriously what it says about itself.

There's something called the 'principle of charity' that operates in debates, and it's about representing your opponent in the best possible way. It's like a top football team – it's no good playing an amateur Sunday league side. They want to face the very best opposition there is, because that's what tests their own knowledge and abilities – and when they win, it confirms they're the best.

If we're Christians, we still believe our faith is true. There shouldn't be any doubt about that. We say God was incarnate in Christ alone – a huge claim.

But that should give us enough confidence not to worry about acknowledging the good in other faiths. We don't have to demonise them or misrepresent them. We can acknowledge where we've got it wrong, too – and put our minds to sharing the love of Christ, and raising him up rather than tearing others down.

Follow Mark Woods on Twitter: @RevMarkWoods