The EU Referendum is the first opportunity British Christians have to vote in a political era that the journalist Faisal Islam has rightly described as 'post-truth'.

The high priest of post-truth politics, of course, comes from across the Atlantic in the form of Donald Trump; a leader so convinced of his own powers that he simply states things he wants to be true, knowing them to be anything but. Yet the post-truth virus has caught and infected British politics to a degree which is inflaming the moral decency of our more decent political leaders on both sides of the spectrum. For Christians it must be recognised as the corrosive and dangerous force that it is. Ignoring it or simply excusing it on the cynical basis that 'all politicians have always been untruthful' would be an act of profound irresponsibility on our part.

It is true that we are used to political leaders who bend and twist the truth like a salesman. That is business as usual. But the open fabrication of statistics, the invention of events and the evocation of non-existent enemies is an altogether more worrying innovation in our public life.

For decades our political system has operated on the basis that the cost of a lie being discovered is too great to the personal reputation of a politician to bear. Arguments have had to be, well, arguable. Politicians have effectively bound each other to this principle with the help of the press. It is a form of self-regulation which has protected everybody and ensured that many good people have defied the cynics and found their way into public life.

Trump's progress has held open to unscrupulous politicians the tantalising possibility that truth is no longer necessary for political gains to be made. The current EU Referendum debate must be approached with this in mind. We must root out arguments made in bad faith and those which represent outright lies. We could reasonably be expected to discount the broader arguments made by politicians who engage in lying. After all, a lie is the ultimate capitulation which admits that the underlying position is indefensible

Into this first category of 'bad faith' we can put the argument made by the Leave campaign that the UK can exit the EU without triggering a major economic slowdown.

The evidence for a slowdown in the event of Brexit is overwhelming. It is the view of every major economic institution which has analysed the problem. So overwhelming is the evidence, that Brexiteers have resorted to questioning the integrity of the most independent and apolitical institutions in our society – and the governments of our closest allies – in a bid to dismiss it. Attacks have rained down on the Governor of the Bank of England, the internationally-acclaimed Institute of Fiscal Studies, the International Monetary Fund, the OECD, the Federal Reserve, the Bank of Japan and the US President amongst others. All of these have variously been attacked by very senior Vote Leave politicians with bizarre insinuations that they have their hands in some unspecified trough, or using language bordering on racism.

True there are some economists arguing for Brexit but these are principally the fringe agent provocateurs who earn their living on the after-dinner speaking circuit around the City. They do not get paid to argue in favour of consensus. Their pay depends on their capacity to challenge orthodoxy and stir debate, regardless of whether that orthodoxy is correct.

The leading figures in Vote Leave know they have lost the economic argument, which is why they have moved the debate so deliberately on to immigration.

The UK economy today is already slowing as a result of Brexit fears; this is evident in surveys that demonstrate companies pressing the pause button on hiring. Such a slowdown is a profound risk to the UK. This is particularly true because the positive economic growth we have seen in the past two years has been driven by falling unemployment, not productivity growth. As an engine for economic growth, falling unemployment only works for so long. This combined with households that are still highly indebted makes for an economy which is growing but with huge in-built fragilities. A shock such as Brexit would take a heavy toll. This would likely be exacerbated by rising interest rates to counteract the inflation created by a weak pound. Add into the mix a significant current account deficit and you have a noxious cocktail.

It is not scaremongering to suggest this mix of factors would wipe percentage points off economic growth; it is about as close to a fact as it's possible to get in the grey world of economics.

It is hardly new, though, to have political leaders arguing in bad faith about the economy. What is new is that this referendum has been marked by the new phenomenon of stating outright lies. As Christians, we know lies corrode and destroy everything they touch and whatever our political views we must stand against them.

So far this 'post-truth' approach has come largely from the right. The most outrageous examples being the claim on the side of the Vote Leave bus that the UK 'sends £350 million to the EU each week', or on posters that say 'Turkey is joining the EU'.

This departure from reality has rightly been identified by eminent centre-right figures as poison for their cause. Sir John Major's furious attack on it was more than just political, it was real anger driven by the knowledge that when truth is abandoned a movement starts to die.

Those of us on the left, though, cannot be complacent. History teaches us that political leaders who live in a fantasy land can emerge from either side of the political spectrum. True it is very rare to see this phenomenon in a democracy, but here it is nonetheless. As Christians we must remember that our commitment to truth and honesty comes ahead of our commitment to any political cause we choose to pursue.

Charlie Parker is a member of christiansontheleft.org.uk and is the director of a London-based investment management company. He is a trustee of the Bear Church and worships at Christ Church Abingdon in Oxfordshire.