Not only ethnic, national and racial diversity but religious diversity too.
On all fronts, the war evidently involved variations of race, nationality and ethnicity.  Diversity of religion was also very evident throughout the war, though until recently less remarked upon.
Europe predominantly but not exclusively Christian
In 1914 the population of countries within Europe was predominantly Christian, with a minority Jewish presence, itself often the butt of prejudice. Secularist rejection of religion was clear, but the greatest diversity remained between different versions of Christianity – Catholic, Orthodox and Protestant and variants within them. Islam was more obviously practised in countries on the other edges of the Mediterranean in N Africa and Turkey. There were individual Muslims within the UK, as also Buddhists, Hindus, Sikhs and Zoroastrians but few in overall numbers.
Other Continents
Christianity was also present in many other countries throughout the world, the result of its historical spread  over 2000 years and more recently from its missionary accompaniment of European empire building in Africa, the Americas, Asia and the Antipodes. The population of the undivided India was mainly Hindu, Muslim and Sikh, plus minorities of Buddhists, Christians, Jains, Jews and Parsees. In China, traditional Folk Religion predominated with Buddhist, Confucian and Daoist ingredients, as similarly in Japan with strong Shinto variations.

'Guesstimated’ figures for religious allegiance in 1900      
CHRISTIAN                 558,132,000 
     Roman Catholic      266,500,000
     Protestant               103,000,000
     Orthodox                 115,000,000
     Anglican/CofE           30,500,000                              
   
MUSLIM                      200,000,000
    Sunni                       173,000,000
    Sufi                            80,000,000
    Shia                           26,000,000

HINDU                        203,000,000
   Vaishnavite              143,000,000
   Shaivite                     53,000,000

NON-RELIGIOUS/         3,250,000
         ATHEIST

BUDDHIST                  127,000,000
   Mahayana                  71,500,000   
   Theravada                  48,000,000   

CHINESE FOLK-        380,000,000 
       RELIGIONIST 
JEWISH                        12,300,000

SHINTO                          6,720,000

SIKH                               3,000,000

JAIN                                1,323,300

ZOROASTRIAN                108,490

CONFUCIAN                     640,050

DAOIST                             375,000

These are approximations of the global adherents of the world’s religions in 1900 as estimated in David B Barrett, et al World Christian Encyclopedia: A comparative survey of churches + religions in the modern world (Oxford UP 2001) vol 1, table 1:1.