Pope Francis and Donald Trump are having a bit of a spat. The Pope told reporters on the plane back to Rome after his Mexican trip that Trump was "not Christian" for wanting to build a wall between Mexico and the US. Trump was annoyed and it was a gift to journalists and social media. Was the Pope right to get involved in US politics? Was it fair to cast aspersions on someone's faith? Would it harm Trump's chances in South Carolina?

It's all a bit predictable. What's interesting, though, is what it reveals about how people talk about religion in different cultures and contexts, and what vast possibilities for misunderstandings there are when we don't understand each other.

Because Francis wasn't, in his own terms, saying that Trump was not a Christian, in evangelical terms, because he wants to build his frontier wall. When he said: "A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian," he was saying Trump was not behaving in a way appropriate for a disciple of Christ. He was not placing him on a par with an atheist or making a statement about his eternal destiny, though some of Francis' Republican critics assumed he was doing exactly that.

What we have are two different uses of the word "Christian". One is describing a type of action, as we might say, "That's not a very Christian thing to do." It's about a generally shared moral outlook characterised by kindness, generosity and charity. The other is more precise, defining who is able to use the title; and in evangelical terms, this is someone who has put their trust in Jesus Christ as their saviour and Lord, which Trump assures us that he has done.

So is Trump a Christian? That's between him and God, though most evangelicals seem happy to take him at his word. Are Trump's behaviour and opinions Christian? There's much more room for debate about that, and it's there that the Pope pitched in. A great deal could be said about his plan for a 2,000-mile-long wall along the southern border of the US. Critics have pointed out that it wouldn't work and would cost billions of dollars. It would also do permanent damage to US-Mexico relations and is a populist attempt to get votes from people who haven't really thought it through but know they don't like migrants. In the light of this, saying it's not very Christian seems rather mild.