The reason so many Hindutvadis are Atheists or not especially religious is because Hindutva has always been a form of ethnic nationalism, it's not a religious movement. It's why they avoid faith and focus on "culture" when arguing against Indians converting to Christianity.

Some may argue you can be a Hindu Atheist. That's true. But mainstream Hinduism of the masses has always been about devotion to God(s), and had Atheistic Hinduism ever became mainstream it would probably lead to the disappearance of Hinduism. Cultural faith is weak and transient.

This is also why despite Hindutva's growing influence there's almost no mainstream talk about what a Hindu economy, society, government, legal system looks like.

Hindutva was designed as ethnic nationalism for an India where European linguistic nationalism wasn't possible.

In that sense Hindutva is more similar to the some forms of European new nationalisms where Christianity is seen as a symbol of culture or an instrument for promoting the "right" values but definitely not the core of the ideology.

Most Hindutva supporters are religious though. Though most Indians of any political ideology are religious cause India is one of the most religious countries on Earth.

But Hindutva intensifies Hindu identity but not faith, unlike most forms of political Islam which do both.