In the UK, the United States president is dismissed or condemned from almost all quarters, and most of us struggle to get our heads around the idea that millions of people voted for Donald Trump. One significant group of these voters were white evangelical Christians. According to the exit polls from the 2016 US election, white “evangelical Christians” voted for Donald Trump over Hillary Clinton by 81% to 16%.

It has not been unusual for presidential candidates to court the so-called “evangelical Christian vote” in recent history. If you vote for a president, it doesn’t necessarily mean you support everything they stand for. I recognise that: the 81% represents a whole host of motivations and degrees of support rather than a united mass gathering under Trump’s banner. But it’s still a big number.

What matters more, the identity of the person in the White House or the promotion of the good news about Jesus Christ?

One story I stumbled across seemed laughable at first. There is a group in the US who brought out a film shortly before the 2018 midterm elections called The Trump Prophecy. This film likened the US president to Cyrus, the first emperor of Persia – a pagan used by God to bring about his purposes.

Throughout the Bible, God uses flawed people, sometimes extremely wicked people – and often these purposes will include bringing judgment on his people. Sure, Cyrus was responsible for freeing the Jewish population from captivity in Babylon, but I think I missed the bit of the Bible where God’s people campaigned to be subject to his rule in the first place. It all sounds like an excuse for people desperate to justify voting for someone who Christians should be rebuking, not supporting.

The Bible is clear that we don’t know the mind of God – that is beyond us; second-guessing him is not something we are encouraged to do. Only he has the big picture. But one thing I know for sure is that we are instructed to love God and to love our neighbour – and that means that someone who separates children from their mothers, bans Muslims from entering his country, and countenances torture, is worthy of rebuke.

So not only is it wrong to support Trump on this basis, it is also dangerous. Dangerous because it contributes to the tribal politicisation of evangelical Christians.

The right is often found decrying identity politics, but is it possible that they are as guilty of it as anyone else? In 2000 and 2004, the evangelical Christian movement in the US allowed itself to become politicised as never before, as it lined up behind George W Bush. Though Christians’ association with Bush was counterproductive, it was, however, understandable. He was a professing evangelical Christian who sought to live in accordance with the Bible’s teaching.

But evangelical Christians’ political marriage to Trump is much harder to fathom. In his private life the president has demonstrated a flagrant disregard for the dignity of others, an utter contempt for equality, and far more concern for his own rights and reputation than the wellbeing and care of others. His attitude to refugees and migrants demonstrates a lack of compassion and understanding for the vulnerable that at times beggars belief.

The politicisation and tribalism of Christianity is dangerous and, in the case of Trump, stands in direct opposition to the values of the saviour who Christians seek to follow. One who gave up his rights to save others rather than trampling on the rights of others to promote himself. Some Christians may believe they are bringing about God’s purposes, but in reality they are causing his name to be associated with the very things he stands against.

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Indeed, it is possible that this association with Trump’s loveless, graceless amoralism could write Christianity out of the public square even faster than the dominance of secularism might on this side of the Atlantic. Where a Christian worldview is dismissed and derided in the UK, association with Trump means many may actively reject it in the US. In fact, the impact of the politicisation of evangelical Christians is not just damaging to Christianity in the US. Churches in Britain have been taking the word “evangelical” out of their name for fear of the association.

The choice of American Christians to publicly back Trump now, and George W Bush before him, has been a dangerous move in a culture war that now means that half of the US has its fingers in its ears when it comes to the gospel. My challenge to Christians in the US is this: what matters more to you, the identity of the person in the White House or the promotion of the good news about Jesus Christ? If it is the latter – and it surely must be – then you need to seriously reconsider your support of the former. The politicisation of US Christianity has undermined the witness of evangelicals everywhere.

• Tim Farron is MP for Westmorland and Lonsdale and former leader of the Liberal Democrats