‘Secularism' basically is a western concept that separates government institutions and functionaries from religious institutions and dignitaries.
It basically means that the state has no religion of its own and that it will neither influence religious institutions nor will get influenced by them.
The constitution that India adopted post independence did not include ‘secularism' in its Preamble. After the country was partitioned on the basis of religion, nobody thought that religion will become a divisive factor ever again.
But things obviously did no go the way they were expected to. It was only in 1976 that the word ‘secular' was inserted in the Constitution through 42nd amendment.
The emergence of BJP in 1990s as a serious challenger to Congress party's hegemony made the secularism vs communalism debate even bitterer.
Riding on the Ram Janmabhoomi movement, BJP grabbed power in UP and emerged as an alternative to the Congress at the national level, with sadhus in saffron robes elected to the portals of Parliament on BJP ticket.