Last year at about this time, megagiant coffee chain Starbucks got into a heap of trouble with America's culture warriors.

For reasons not readily apparent, it has got social media to do most of its Christmas marketing for it by making a thing of its seasonal cup design – red, with a vaguely Christmassy theme. Last year the hyper-vigilant evangelical right went a bit crazy, because it was just a red cup; Starbucks was trying to ban Christmas and turn America secular, etc. In the UK the seasonal victim is Royal Mail, which is criticised every time it issues non-religious Christmas stamps.

This year, though, Christmas has come early for Starbucks. It's released a cup in green, featuring the faces of more than a hundred people, drawn with a single continuous line. "The green cup and the design represent the connections Starbucks has as a community with its partners [employees] and customers," Starbucks CEO Howard Schultz said. "During a divisive time in our country, Starbucks wanted to create a symbol of unity as a reminder of our shared values, and the need to be good to each other."

It's rather nice, and a considerable artistic achievement. But social media has been up in arms – it's green, not red, and it's too liberal lefty feelgood, rather than being, well, Christmassy.

Critics have, of course, pointed out their mistake – that it's an election cup, not a Christmas one, which is being released shortly. To avoid buying into the Starbucks marketing machine, I'm not including a preview pic.

To be fair, there are more people gloating about how Christians are going to feel about the #greencup than Christians actually complaining about it. But the row – confected as it is – says something sad about just how marginalised many US Christians feel, how toxic the evangelical label is, and how far off from the mainstream of US political life they've become. The assumption is that Christians are going to be furious about the colour of a cup. Why? Because they're furious about everything that seems to encroach on the Christian identity of America. Every gnat has to be strained out.

It's not a fair characterisation. But in this frenetic election season, hyperbole and vitriol are everywhere, and Christians are in it up to their necks. Facts are secondary, opinion is everything. Things are written that should never have been written. Reaction comes first, reflection second. The opponent is the enemy, and the enemy is to be crushed, not won over. It's as though we're in a dystopian other-world, like the film The Purge – periodically the laws of right and wrong are suspended, and everyone can do what they like.

I don't live in the US, but I know the temptation. I find it desperately hard to see any moral or intellectual virtue in people who voted for Brexit. But what keeps me generally polite and respectful is the fact that I know some of them personally. They aren't just symbols of a mindset I abominate; they have reasons, even if I'm not convinced by them. It's that failure to encounter real people who think differently from us that drives people to say and do things they later feel terribly sorry for – one reason why the message of Starbucks' green cup is actually right. 

So here are five questions I'd like to suggest those tempted to engage in social media warfare ask themselves.

1. Are you sure of your facts?

Don't blast an opponent just because they represent a position you don't agree with. Look at what they're saying. Play the ball, not the man.

2. Is what you're about to write kind?

Politics is a rough game, and to a degree it has to be: ideas that aren't tested to destruction probably aren't very good ones. But will your contribution help destroy an idea – or a person? The former's fine; the latter, not.

3. Could you say it to their face?

Social media is wonderful. It helps us share, engage and change the world, at least in theory. It's also a refuge for cowards, a digital foxhole that lets us fire off our prejudices and abuse without having to look someone in the eye. A post is not just a post, it's an encounter.

4. Do you need to say it?

One of the things that leads us to say things we shouldn't is the need to have the last word. It wastes time and spiritual energy.

5. Does it commend Christ?

If you're identifiable as a Christian online, everything you say speaks of Christ. So don't hate, don't misjudge, don't like and share just because someone else does. Don't let your anger lead you into sin.

Follow Mark Woods on Twitter: @RevMarkWoods