Anyone who's ever preached a sermon knows the familiar struggle only too well. You've been handed a particularly challenging passage in Lamentations, from which you're expected to bring forth a message of light, life and hope, and you really need a way in. You're desperate for a story that will grab your listeners' attention, possibly provoke an audible gasp of amazement, and hopefully teach the main point of the Bible reading far better than your ham-fisted exegesis ever could.

In your moment of need, you come across a Story On The Internet. The source is wonderfully vague, but it's just too perfect a fit to ignore. Without a moment of fact-checking required, you copy and paste it on to your opening page. Your sermon is saved!

Or perhaps you can't find a story that quite fits, but you're reminded of one which almost works. Maybe it's a situation you experienced in the past, or something you heard second or third-hand. Presented unfiltered, it doesn't quite teach what you want to get across, but with a little bit of tweaking – or perhaps conflation with another completely separate story –bingo. While it sounds like a fairly dodgy practice when you put it so brazenly, it happens all the time. Embellishment, deliberate or otherwise, is an all-too-regular feature of the modern sermon.

Sometimes it's a beloved story, passed down from preacher to preacher without ever passing through the filter of fact-checking. For instance, you've almost certainly heard at least one sermon which draws on the story of missionary coloniser and explorer Hernán Cortés, who famously 'burned his boats' when he arrived at Mexico. The story goes that he torched his entire fleet in order to demonstrate his resolute commitment to never turning back, and the tale has been appropriated by countless preachers as an illustration of 100 per cent commitment to Christian mission. Yet the truth is that Cortés didn't actually burn his boats. According to historians, he stripped them for wood and other building supplies in order to create dwellings in the New World, and he left at least one completely intact so that gold and other treasures could be sent back to Spain, and crucially, so that the officers could retreat if things went badly wrong.

Why has that false story thrived for so long? Partly because the rumour about burning boats spread in Europe long before any preacher decided to make use of it, but partly because it's just more perfect this way. It makes Cortés into a hero and a role model for Christian leadership. We really want stories like this to be true, and so we make them so. While the truth about Cortés has been widely known for years, the story still abounds – it'll probably feature in at least one sermon this weekend.

While this kind of embellishment is at least partly unintentional, another is far less forgivable, and almost certainly as common. The rewriting, mixing up and complete invention of stories to fit our purposes might feel like a necessary evil in the fight to inspire and encourage our listeners, but as communicators we have to call this what it really is: lying. If we take two stories, mash them together and make one, we're lying when we tell it as a single tale. When we improve the scale or the ending of a story to make it seem more impressive, we're lying from the moment we move the goalposts. It can't be justified by noble intentions; telling embellished stories is misleading and wrong. How can we possibly expect God to speak truth through our lies?

It seems to me that this issue is connected to another practice, that of exaggerating impact (or 'evangelasticity' as I termed it in another recent article). In both cases, what we're really saying is that the truth about God, or what he has done around us, is not impressive enough to tell unfiltered. We seem to think that by 'enhancing' a testimony, we somehow glorify God more. Nothing could be further from the truth. When we stop to think about it, do we really believe that the guy who invented the Northern Lights, the rings of Saturn and the Grand Canyon really needs our help with the spectacular?

So come on, let's all commit to eradicating this practice from our communication. Let's fact-check stories before we use them (the more remarkable they seem, the more you need to check), and let's not be tempted to make a story just a little bit more perfect for the sake of illustrating a point that the Bible probably makes perfectly well on its own. Embellishment and exaggeration are easy traps for preachers to fall into, but that doesn't make them right.

It's better actually to be the people of the Truth, than to be able to tell the perfect story.

Martin Saunders is a Contributing Editor for Christian Today and the Deputy CEO of Youthscape. Follow him on Twitter @martinsaunders.