What is a chaplain actually for? It is a question that was to be asked very sharply of those who served in the First World War. Of course, they would take services on Sundays, with compulsory attendance. They would be available for men to talk about their worries, at home and on the battlefield. They would pray for the wounded and dying. It often seemed to them, though, that their main ministry was purely practical: writing letters, passing out cigarettes, putting on concerts and generally cheering the men up. A demoralised soldier was no good in the line, after all.

Set against the carnage of the trenches, it sometimes seemed that they had very little to offer. Siegfried Sassoon in his 'Memoirs of an Infantry Officer' describes a casualty clearing station full of desperately wounded men. The only person there who seemed to have nothing useful to do was the chaplain, "discovering the inadequacies of the ministry of the Church of England".

The most challenging description of the chaplain's role is in CE Montague's famous memoir, 'Disenchantment'. His chapter, "The Sheep that Were Not Fed" is a classic, for uncomfortable reasons. He does not paint a flattering picture of the average chaplain. There were rogues and inadequates among them, like the chaplain "drunk at dinner in Gobert's restaurant in Amiens on the evening of one of the bloodiest days of the first battle of the Somme". On the other hand: "There was the man who, urged by national comradeship, would have been a soldier but that his bishop banned it: to be an army chaplain was the next best thing. There was the man who, urged by a different instinct, felt irresistibly that at the moment the war was the central thing in the whole world and that it was unbearable not to be at the centre of things. And there was, in great force, the large, healthy, pleasant young curate ... He abounded so much that whenever now one hears the words 'Army Chaplain' his large genial image comes to mind." Montague's view was that the chaplains were given the opportunity to minister at a time when men were particularly receptive to spiritual things, and that most of them found that they had nothing to say.

This was not entirely fair. This was a war like no other, and these men – many of them young, often of the middle and upper middle classes, educated at public schools and Oxbridge – were faced with ministering largely to working-class men who were indifferent to religion anyway. They had to find something meaningful to say in the middle of a conflict in which the usual comfortable words were meaningless.

There were certainly some who rose to the challenge. T B Hardy, for instance, was killed in 1918 after having won the MC, the DSO and the VC. Over 50 when he joined up in 1916 and a school headmaster, Montague calls him a "hero and saint". He spent as much time as he could in the front line and once stayed for 36 hours with a soldier sinking to his death in the mud from which no one could get him out.

Another is the famous G A Studdert-Kennedy. Known as "Woodbine Willie" for his habit of handing out packets of Woodbine cigarettes, was a charismatic figure who succeeded in bridging the gulf between religion and the soldier. He served on the Western Front in the worst of the fighting and was awarded the Military Cross for his courage in rescuing wounded soldiers. He was not popular with the top brass – a general once walked out of one of his sermons – but the men loved him. He was a fine orator who after the war could fill huge venues, and a poet who wrote, sometimes shockingly, of the effect of the war on the human spirit. Most of all, he was consumed with the love of Christ, who suffered with all human suffering. He died in 1929, worn out with over-work, and his funeral was attended by thousands of old soldiers.

Another famous chaplain was P B "Tubby" Clayton, who founded a soldiers' club in Poperinghe, just three miles behind the lines at Ypres. A sign over the door read "Abandon rank, all ye who enter here". It was an oasis and a home from home, with a chapel on the top floor which saw hundreds of communions with men who were soon to face the enemy again. It was bought after the war and preserved as the mother house of the Toc H charitable fellowship (the name is from the signallers' code for the club, named Talbot House).

In 1973 the journalist Michael Moynihan collected unpublished accounts the war and published them in "People at War 1914-18". One of his subjects was the Rev John Michael Stanhope (Repton and Brasenose, Oxford) who left his rectory in Kettlesthorpe, Lincs (pop. 400) to go to a Casualty Clearing Station on the Somme. He wrote home every day, sometimes, exhausted, at 1 or 2 in the morning. He saw terrible things, and at times it seems that the dying were passing him almost as if on a conveyor belt, leaving him time to do very little. In one note he writes: "But I always feel if I tell these things dear people say, Oh how splendid, isn't he cut out for the job? That's the trouble, he isn't. Or, doesn't he do good, see letters of relatives, especially officers. No, it amounts to this – one has an opportunity, the war is putting them into your hands, so to speak, what am I doing with them? Far less than the average priest would do – I can swear to that. I am not any good at this sort of thing. I can't help them clutching at any straw and clutching me."

After the Somme, he went home. "No, it is not worth staying out here whilst one has children at home. Tommy does not want religion. I don't persuade him." He left with a certain sense of failure.

All of them, in their different ways, faced the same challenge: what did it mean to be the face of the Church, to incarnate the presence of Christ, in a situation where every sinew was strained in the most un-Christian of activities – total war?

To a certain extent, it's the same today. There are chaplains in every sphere of military activity and some of them have behaved very bravely, sharing the risks of soldiers in Iraq and Afghanistan. But today, the chaplain's resources are far greater. They know what they are getting into. There are codes of ethics, there is training and support; they know what the job is. In the Great War, they did not.

In his biography of Woodbine Willie, William Purcell says: "They were the first of their kind ever to be involved in a situation of total war, and they went untrained and unprepared from the parochial round, the common task."

But as the Archbishop of Canterbury, Randall Davidson, said: "It is only a very limited number of men, of any sort of class or profession, who would be fit at such a juncture to do what was needed."