How a religion is manifested? Lets examine by the size of the groupings(sure with some overlappings):

1)Small groupings:a)The aspiration of the founder God/gods/goddesses/messiah/prophet....b)Power of the top clerics c)Genuine Philosophers/moralists d)Ruling elite allies who see advantage of joining.....

2)Medium size groupings: a)Military wing like crusaders, jehadists.....b)Livinghood of the average clerics.

3)Large groping--the average adherents

***Internally manifested as group behaviors/psychology/symbolism----a)In more positive language, it's called cultural heritage   b)In less positive language, its called tribalism.

***Geopolitically, it's vehicle of tribal/ethnic/racial  power,influence...like the man who had the last say on the future of mankind is an arab....and arabic is the language of Jannah/paradise

(A young man who became the pastor of a small or medium size church in his 20s and stayed in that positon,say, for 15-20 yrs would understand(but seldom publicly expressed)the above very well, he'll find it difficult to switch to a different career)

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We live in a fast changng world but

Sunni Islam is the only big civilisational pillar of Arabia

Hinduism is the only big civilisational pillar of India.

**Slavery was abolished in Saudi Arabia in 1962---Why so late?Well, the Holy Prophet Mohamed(PBUH) kept slaves

**What hold india together? Are the majority of indians of the same race, same language...same historical lineage?

  You gotto be kidding; but ahha, 80% of indians are hindus.

I call it civilisational fragility if a nation depends so much on a single religion because any attempt to change amounts to controversy if not blasphemy. The Euros have the Greek-roman assets to fall back on. And we know confucism is only one of a number of chinese(political or not) ideologies

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The jews are a partial exception because most of the jewry live outside Israel and they got to adapt to survive like a)57% of US jews eat bacon b)The black dress of hasidic jews actually has polish origin  c)Karl Marx was a jew too, right?  Obviously 'Judaism' is more a racial/ethnic tag to the jews than a 'religion'