A perspective from on high one Sunday morning
"Up there, from the sky, you don't see much, you know. Among the squares of the fields and the little heaps of the villages the roads run like white cotton. You can make out, too, some hollow threads that look as if they'd been traced with a pin-point and scratched through fine sand. These nets that festoon the plain with regularly wavy marks, they're the trenches. Last Sunday morning I was flying over the firing-line. … I could make out two crowds, one among the Boches, and one of ours, in these parallel lines that seemed to touch each other; each was a solid, lively lump, and all around 'em were dots like grains of black sand scattered on grey sand, and these hardly budged--it didn't look like an alarm! So I went down several turns to investigate.
Then I understood. It was Sunday, and there were two religious services being held under my eyes--the altar, the padre, and all the crowd of chaps. The more I went down the more I could see that the two things were alike--so exactly alike that it looked silly. One of the services--whichever you like--was a reflection of the other, and I wondered if I was seeing double. I went down lower; they didn't fire at me. Why? I don't know at all. Then I could hear. I heard one murmur. one only. I could only gather a single prayer that came up to me en bloc, the sound of a single chant that passed by me on its way to heaven. I went to and fro in space to listen to this faint mixture of hymns that blended together just the same although they were one against the other; and the more they tried to get on top of each other, the more they were blended together up in the heights of the sky where I was floating.
I got some shrapnel just at the moment when, very low down, I made out the two voices from the earth that made up the one--'Gott mit uns!' and 'God is with us!'--and I flew away."
The young man shook his bandage-covered head; he seemed deranged by the recollection. "I said to myself at the moment, 'I must be mad!'" …
With his eyes shining in delirium, the narrator sought to express and convey the deep disturbing idea that was besieging him, that he was struggling against.
"Now think of it!" he said. "Fancy those two identical crowds yelling things that are identical and yet opposite, these identical enemy cries! What must the good God think about it all? I know well enough that He knows everything, but even if He knows everything, He won't know what to make of it."  
Extract from Henri Barbusse Under Fire: The Story of a Squad (ET 1917) – a philosophically reflective novel by an author with 18 months experience on the Western Front recounting the view of a wounded pilot. http://www.ibiblio.org/eldritch/wwone/uf21.html and http://www.gutenberg.org/files/4380/4380-h/4380-h.htm  
Matching prayer books
"The German prayer book found in the pillbox" at Poelcappelle - one the cover 'Mit Jesus in der Feld'
and inside a coloured picture of Christ looking with pity on a dead German soldier. The same picture as in our own Field Prayer Book., 'With Jesus in the Field'. The only appreciable difference being in the uniform of the dead soldier - German in one khaki in the other - and the feeling experienced on seeing that picture compared with one of our own, that the whole was was a horrible mistake." Quoted by Niget Steel & Peter Hart  Passchendale Thr Sacrificial Ground (2001) p 331 from the Diary of Sydney Fuller (Signalller Suffolk Regiment) as held in the Imperial War Museum https://www.iwm.org.uk/collections/item/object/1030000126
Double vision from Berlin and Canterbury
“A fateful hour has fallen upon Germany. Envious people on all sides are compelling us to a just defence. The sword is being forced into our hands…war will demand of us enormous sacrifices in blood and great treasure, but we shall show our foes what it means to provoke Germany. And now I command you all to go to church, kneel before God and pray to him to help our gallant army.” Kaiser Wilhelm’s address to the German people from the balcony of the Imperial Palace in Berlin August 1^st 1914. “At the Cathedral near the Palace, the Dom, a special service commenced at 6pm as the Kaiser’s pastor intoned the words from Psalm 130 ‘ Our of the depths I cry to you, O Lord, hear my voice, Let your ears be attentive to the voice of my supplication.’ At the new synagogue on Oranienstrasse, a Rabbi led the congregation in prayers for victory in the coming struggle.” Laurence V Moyer Victory must be ours. Germany in the Great War 1914-1918 (1985) pp 72-3, citing the Berliner Tageblatt. 
King George V Aug 4^th The King has sent the following message to his Colonies: "I desire to express to my people of the overseas dominions with what appreciation and pride I have received the messages from their respective Governments during the last few days. These spontaneous assurances of their fullest support recall to me the generous, self-sacrificing help given by them in the past to the mother country. I shall be strengthened in the discharge of the great responsibility which rests upon me by the confident belief that in this time of trial my Empire will stand united, calm and resolute, trusting in God." On August 2nd the Archbishop of Canterbury, Randall Davidson, preached "What is happening  is fearful beyond words, both in actual fact and and in the thought of what may come to be...This thing which is now astir in Europe is not the work of God but the work of the devil." (Quoted by Alan Wilkinson The Church of England & the First World War p 18.) With the agreement of the  Free Church Council and the leaders of the English Roman Catholic Church special national services were arranged on Friday 21 August. (http://dro.dur.ac.uk/10638/1/10638.pdf)
Christian vitriol from the USA
https://theconversation.com/how-christianity-shaped-the-experience-and-memories-of-world-war-i-75449
A Historian’s judgement
“..the war drew attention to the hold Christianity appeared to possess over the passions of multitudes or the actions of their governments. European Christianity, supposedly based on a common moral foundation, proved no more able than the network of marriage relationships among the royal families to prevent the Armageddon, or to stop it degenerating into mutual genocide. The doctrinal and ecclesiastical divisions of Christianity, so stridently debated and defended, proved equally, if not more, irrelevant. All the participants claimed they were killing in the name of moral principle. All in fact pursued purely secular aims. Religious beliefs and affiliations played no part whatever in the alignments. On one side were ranged Protestant Germany, Catholic Austria, Orthodox Bulgaria and Moslem Turkey. On the other were Protestant Britain, Catholic France and Italy, and Orthodox Russia. Thus divided, the Christian churches could, and did, play no part in transcending the struggle and bringing about reconciliation. Clergymen were unable, and for the most part unwilling, to place Christian faith before nationality. Most took the easy way out and equated Christianity with patriotism.” Paul Johnson A History of Christianity (1976) p 477