Trump tweets. It's what he does, and surely no one in their right minds would pay any attention. The man lacks any kind of filter. That mental watchdog that keeps most of us from giving voice to the random thoughts that drift over the surface of our minds spends most of its time, in his case, asleep. It's unfortunate that modern communication technology has allowed him to broadcast his thoughts instantaneously to a waiting world, but still – does it really matter?

In general, no. Who cares about his attacks on journalists or other individuals who've irritated his extra-thin skin? But sometimes, yes.

When he retweeted Britain First videos purporting to show Muslim outrages against Christians, he crossed a line – and everyone who isn't paid to support him knows it. Here's why.

1. The source is contaminated

Perhaps Trump was drawn to Britain First because it echoes his own 'America First' slogan. Whatever you think about the latter, the former is a xenophobic, hate-filled organisation that co-opts Christian symbols and language for a devilish purpose. Its 'Britain' is not one decent British people recognise. It's no coincidence that the murderer of Labour MP Jo Cox shouted its name when he killed her; anyone trying to bring communities together, as she did, rather than set them at each other's throats, is anathema to these people. When Trump retweeted their videos, he was tacitly approving of their rhetoric, their values and their aims – and they are all poisonous.

2. The message is wicked

Let's be clear: the videos are unverified. No one knows where they're from or whether they portray what Britain First says they do – Muslim attacks on Christians and Christian symbols. But just suppose, to strain at the limits of generous tolerance, they are what they say they are. The message of these videos is that this is what Muslims do, because they are Muslims. Never mind that Muslims are first and foremost people, with a fair mixture of good and bad like every other people group: these videos are designed to make people hate. They're meant to make us believe Muslims are evil. They aren't.

3. The ground is fertile

All over the world, nationalist and racist movements are stirring and growing stronger. Hindu fundamentalism is on the rise in India. We know what Buddhists in Myanmar are doing to the Rohingyas. Muslims are killing Muslims in Egypt, as well as Christians. In Europe the far right is on the march: Alternative fur Deutschland in Germany, Jobbik in Hungary, Golden Dawn in Greece and Austria's Freedom Party. Dig deep enough and the same instincts are in play: fear of a wider world, a desperate need to scapegoat someone else and a willingness to label the 'other' as the enemy. British First is a textbook case. Trump has played into its hands.

4. There are consequences

For the President of the United States – even when it's Trump – to be complicit in telling people it's OK to hate Muslims is serious. It affects real people's lives. It gives people permission to be violent, to discriminate, to exclude. And in the wider world, it skews diplomacy and makes for hostility and suspicion rather than trust and openness.

Trump has done a bad thing. Would he have retweeted a video from the Ku Klux Klan? Perhaps, in that case, the drowsy watchdog might have stirred. But in effect, that's what he's done. Just once, he should admit he's got it wrong. But what are the odds of that?

Follow Mark Woods on Twitter: @RevMarkWoods