THE alleged terrorist plot against air travellers has raised again a question fundamental to the “War on Terror”. How can a religion devoted to peace and justice be used to legitimise the taking of innocent lives?

Since the alleged plot was reported, the answers to this question have been predictable. Islamist terrorists, it has been argued, are not truly “Islamic” at all: they have been poisoned with false ideologies or brainwashed by extremists. Nor, despite their willingness to sacrifice their lives for a higher cause, can they be regarded as idealists. At best they are tragic dupes; at worst they are evil fanatics.

These responses are understandable but profoundly mistaken. By failing to acknowledge the sincere faith of religious terrorists, we risk misunderstanding our enemies. Because we pictured Islamist bombers as cruel fanatics we were surprised to discover that Mohammad Sidique Khan, the ringleader of the 7/7 bombers in London, was a respected youth worker and teaching assistant. It is equally dangerous to imagine that the supporters of “militant Islam” have no rational arguments on their side. Unless we acknowledge these arguments we have little hope of challenging them effectively.

Some reflections on Christian history may clarify this assertion. Our failure to comprehend the thinking of the killers, or even to credit them with rational motives at all, reflects the prevalence of a particular view of religion in the modern west. This sees faith as an essentially private matter, and accepts that a multitude of different beliefs are consistent with a good life on Earth and salvation hereafter.

The dominance of this view is comparatively recent. For much of Western history it was eclipsed by the conviction that religion was a matter of public concern and its truths were universal. Within this tradition, which held sway from the Middle Ages to the Enlightenment, it was common to justify violence in the name of God.

Understanding this tradition can teach us two things. First and most simply, there is nothing peculiar to Islam that encourages the shedding of blood. From the massacre of heretics in 13th-century France to the confessional wars of the Reformation, Christian zeal legitimised the killing of innocent people. This was not because Christianity was a particularly vicious creed — any more than Islam is today; rather, it reflected the universal claims of the pre-modern Church.

The second lesson from history is less palatable. The religious wars of medieval and Renaissance Europe were the logical consequence of the “age of faith”. If the claims of Christianity held true for everyone, then those who disputed these claims were in error; and if salvation depended on accepting the truth, those who erred were either denied salvation or condemned to suffer in Hell. “False” believers became a threat when they sought to convert others to their opinions. In 1584 the English pastor George Gifford captured the gravity of this threat in a simple question: “Who is the worst murderer? He who murders the body or he who murders the soul?"

This thinking did not legitimise indiscriminate violence against false believers: in theory at least, the concept of the just war set limits on the conduct of Christian warfare. But by casting false doctrine as a public menace, Christians presented the destruction of alternative beliefs as a desirable goal: the aim of idealists and, when necessary, pious soldiers. The alternative was “indifference”, a contemporary synonym for atheism.

A sympathetic reading of Christian history suggests that there is nothing inherently immoral about religious violence. Indeed, one could argue that the advocates of religious toleration in the age of faith were less high-minded than their militant contemporaries, since they placed earthly concerns above issues of eternal truth. In our own time, we accept that soldiers may be called to fight for democracy, self-determination and the defence of human life. The advocates of Christian warfare had an inestimably greater cause: the defence of eternal life. We should not be surprised that they were prepared to kill for it.

The recognition of this alternative way of thinking allows us to ask what exactly is wrong with it. This requires Christians and Muslims to address some questions that are often skated over. What balance should be struck between religious toleration and the importance we attach to the universal truths of faith? To accept that people may openly reject the core teachings of any religion and still be saved — and therefore pose no threat to the souls of others — is to accept that these teachings are not essential to salvation. This view may be socially desirable, but it also challenges the idea, held implicitly by many believers, that the doctrines to which they adhere are a pathway to heaven.

The same challenge faces those who uphold the authority of sacred texts. The New Testament is emphatic about the unique role of Christ in the scheme of salvation; the Koran is equally clear that this claim is untrue. The acknowledgement that both Christianity and Islam offer a route to salvation requires that each of these claims be qualified. The challenge for Christians and Muslims is to accept this without diminishing the status of their respective revelations.

These conundrums suggest an unsettling possibility. The thinking of religious terrorists may be perfectly rational and consistent; and their faith may be no less sincere than that of “moderate” believers in the Christian and Islamic traditions. By exposing the ambiguities in our own thinking, the lethal logic of religious violence may force us to construct an equally coherent response.

Darren Oldridge is senior lecturer in history, University of Worcester