As recently as 50 years ago, the general consensus in Britain - as it had been for over a millennium - was that Christianity was self-evidently on the side of the angels. One might not actually believe in it, but it urged people, with its celestial incentive scheme, to be decent, honest and kind.

Today that consensus is dead. Talk of Christian morality is as likely to bring to mind the Crusades and paedophile priests as it is decency, honesty and kindness.

This slide in the ratings has, of course, come from the disintegration of Christianity in Britain. However, post-Christian antipathy to the faith can be as blinkered as any religion, and the propagandists for both sides have been guilty of misinformation and a dizzying amount of spin.

To come to some kind of balanced judgment, we should start with the unavoidable fact that the church has been guilty of the most horrific crimes against humanity. These are not just aberrations and bad faith days, nor simply the work a lunatic fringe or political leaders using religion as an excuse. They implicate some of the church’s most celebrated saints and reformers.

For all the notoriety of the crusades and the inquisition, the greatest sin of the church has been its anti-Semitism. Early conflict between Christianity and its mother faith led to the demonisation of Jews, and as soon as Christianity became the religion of the Roman empire bishops started burning synagogues. Such victimisation continued right through into the 20th century, and was supported by popes and ecumenical councils, and by Catholic, Protestant and Orthodox teachers.

Beyond this, the counsel for prosecution has much else on which to draw: the damage and exploitation brought by Christian colonists like the conquistadors; the moral failings of any given hero of the faith; the wars of the reformation, from 16th century Germany to 20th century Ireland; the mess caused by anti-sex teachings; censorship of art and science.

And yet accounts of the faith that present this as the whole story are equally guilty of distortion. It was Christian faith that drove Martin Luther King's campaign against segregation, just as it drove earlier campaigns against the slave trade and the exploitation of workers.

European monasteries single-handedly preserved the literature and philosophy of Greece and Rome throughout the dark ages, just as Christianity provided a foundation for the arts and sciences in later centuries. Monasteries also provided society’s only schools and hospitals. The first Christian emperor of Rome outlawed crucifixion, the killing of unwanted children, the abuse of slaves and peasants, gladiatorial games, and facial branding, and improved prison conditions. Just provision for the poor has been a constant preoccupation of Christians, from St Paul's collections to the Drop the Debt campaign.

Admittedly, there is a limit to how far we can get with this question by this kind of moral double-entry bookkeeping. There is little sense in trying to calculate whether the abolition of the slave trade is enough to atone for the crusades.

What is clear, however, from even this briefest of surveys, is that there are both colossal moral achievements and colossal moral failures in the church's account. Those who portray it as purely a force for good, or as one long crime, are not telling anything like the whole truth.

The other question we have to ask is how often Christianity has been the genuine motivation for these achievements and failures, and how often it is simply the clothing of what would have happened anyway.

One hears a lot of special pleading on this question from both sides: Christian philanthropists were moved by humanist motives, and just talked about their good deeds in religious language because that was the age they lived in. Or: People burned synagogues were not real Christians, because real Christians could never do such a thing.

The unsurprising fact is that, in reality, it works both ways on both sides. There have certainly been many wars where religion was an excuse for non-religious motives - from the Norman Conquest to the northern Irish troubles. Conversely the sacred artists and architects of the middle ages who were so foundational for modern culture were doubtless not purely motivated by glorifying God. The failures and achievements we are talking about are often the failures and achievements of the people, rather than of the faith.

But at other times Christians’ beliefs have clearly had a profound influence on the way they act. The inquisition was not simply an excuse for killing people the state wanted dead anyway. It was genuinely about rooting out religious error. Equally, Wilberforce’s commitment to stopping the slave trade was unquestionably the result of his prior commitment God.

So, while religion may often be incidental to the moral failures and achievements of Christians, there also many occasions where it has been essential.

Perhaps - though this is untestable - the real effect of Christianity has been to intensify a person's existing leanings. For those who are basically decent it gives a spur to better themselves; for those who are basically callous it spurs in the opposite direction.

Conjecture aside, what is undeniable is that Christianity has a thoroughly mixed record. It has been directly and indirectly responsible for campaigns of devastation and persecution, and campaigns of liberation and justice. It's fair to say that it has not made people as good as it was supposed to, but then a fair assessment has to see such failures alongside its very real achievements.

Stephen Tomkins is the author of William Wilberforce and A Short History of Christianity, both published by Lion Hudson.