In this post-truth age it's been repeated so often that it's become true: evangelical Americans voted for Donald Trump in massive numbers and helped him on his way to the White House.

In actual fact, this ignores something very important about the word evangelical. It refers to someone's background beliefs and social assumptions rather than their practical religious commitment – like being 'Church of England' did not so long ago on the other side of the pond. Among churchgoing evangelicals, Trump wasn't nearly as popular. Furthermore, while 'evangelicals' were helpful, they weren't nearly as helpful as Catholics.

During 2016, though, we learned that facts and truth aren't the same thing. So just supposing it's true that evangelicals got what they wanted when Trump was elected, what does it mean for evangelicalism, and why should evangelicals worry?

1. It means they'll be disappointed. Granted that many evangelicals voted Trump on the grounds that he wasn't Hillary Clinton, there was still a sense ­– fostered by his astute cultivation of high-profile evangelical figures – that his values aligned more with evangelicals than Clinton's did. But a lot of this was about language rather than content (see above) and Trump's actual policies will not give evangelicals everything they want by a long chalk. Even his probable pick for the Supreme Court to replace deceased Judge Antonin Scalia turns out to be flawed: Judge William Pryor backed a revolutionary opinion in 2011 that anti-transgender discrimination qualifies as sex discrimination and so was forbidden under the US Constitution. The row over transgender people's use of public lavatories is a litmus test for orthodoxy among conservatives, and Pryor appears to be on the wrong side. He also backed the suspension of Chief Justice Roy Moore for refusing to remove his monument to the Ten Commandments.

2. It may do their long-term credibility no good. There's every reason, for the good of America and the wider world, to hope and pray that Trump's presidency is a roaring success in home and foreign policy. Given his past pronouncements, however, there's almost every reason to doubt it will be. He has staked out some extreme and compromising positions – for instance regarding Russia and Nato – that leave him open to challenge at every level. The perceived support of evangelicals for such an apparently flawed candidate is a huge gamble. If it pays off – if there is peace in the Middle East, if Vladimir Putin ceases his military adventurism and withdraws from Ukraine, if the US economy – which is actually doing pretty well, with increased employment in every one of the last 75 months – becomes turbo-charged through new trade deals and if Trumpcare turns out to be better than Obamacare – they might be able to claim victory. But hitching their wagon to such an uncertain enterprise is a very risky strategy, and evangelicalism might well get caught in the fallout.

3. It delays real, hard thinking about politics. What's been lamentably clear during the election campaign is that many evangelicals have conflated the Gospel with a particular conservative worldview. They have baptised a political philosophy – and not just that, but particular political policies – and called them Christian, when they are no such thing, any more than any political philosophy or policy is 'Christian'. They might align to a greater or lesser extent with our understanding of how the Gospel works itself out, but Christians have no business lending their names, reputations and arguments to them. Arguably Wayne Grudem did this; arguably Bill Johnson did. And the problem with winning an election is that you think you've won the argument. You think you've gained, not the handful of votes that make the difference between winning and losing, but the hearts and minds of millions, who are in some unspecified way voting for an evangelical expression of Christianity. But it doesn't work like that. Those hearts and minds are still on a long-term trend away from Christian faith. They are still sinners who need a saviour. And looking to politics and politicians to effect the sort of change that can only come through patient, bottom-up discipleship and mission in real communities, through unsung sacrifice and loving faithfulness to Christ.

Political evangelicalism is in danger of forgetting a very old maxim: that when you sup with the devil, take with you a very long spoon. Trump is no more devilish than any other politicians, but that's the point: none of them will do what you want them to do, none of them will bring in the Kingdom of God, and none of them will do the work of Christ for you. That's not their job; it's yours.

Follow Mark Woods on Twitter: @RevMarkWoods