Question

in spite of having a was social diversity what keeps the people of India together

Answer 1

Its because of our constitution amendments, it is written in favor of every religion.

for each religion the laws are different, no one can force anyone to follow any religion.

example :Punjabi's wore a turban so, there is a law for them no need to wear a helmet.
so, police cannot fine them for not wearing the helmet

Answer 2

Yes
the constitution of India makes us a single unit after having this much of diversity
every religion is free to opt and pray their gods
and this is a single secular country with this much of diversity