Why do we call India a secular state and how is it different from anti-religion?

Secularism in the Indian sense does not mean being against religion or having no religion. It means the State treats all religions equally and does not have an official state religion of its own. The government does not favour or discriminate against any citizen on the basis of religion, and every person is free to follow, practise and propagate the religion of their choice under Article 25. This is different from some countries where secularism means the State stays strictly away from religion. India's model allows the State to intervene in religious matters to ensure equality, for example banning untouchability or social evils. So India is secular because it keeps equal distance and equal respect for all faiths, protecting your freedom to worship while ensuring no religion gets special political power.