The way H. Farook wore his atheism on his sleeve worried his father, R. Hameed. But he had never imagined it would come to this. The 32-year-old Farook was recently stabbed to death in Coimbatore for "anti-Islam" views posted on his Facebook page and a WhatsApp group. His attackers were allegedly a group of Muslim youths that included two friends.

"When my family didn't have a problem with him, what right did others have to take offence and kill him?" asks Hameed, adding that he has been a believer and so is Farook's wife, Rasheeda. Hameed is now considering quitting his job and joining the atheist group his son was part of.

Farook, a small-time businessman, is the latest to join the ranks of atheists and rationalists who have been killed in India in recent years, including writers Narendra Dabholkar, Govind Pansare and M.M. Kalburgi. Right-wing Hindu organisation Sanatan Sanstha is said to be behind the three murders. "The attack on Farook completes the cycle if you take into account the hounding of Sanal Edamaruku out of the country by some Christian groups. Fundamentalists hate religions other than their own, but they hate atheists and rationalists belonging to their own religion even more," says Narendra Nayak, a Mangalore-based rationalist, who is under police protection following threats to his life by Right-wing Hindu groups.

Hamid Dabholkar, the late rationalist's son who continues his father's work in Pune, draws attention to an ancient philosophy. Charvaka apparently thrived even though it dismissed concepts such as heaven, hell, God and cycle of birth that could not be proven. "Constructive criticism of religion and superstitions is the bedrock of a great society. Our forefathers were tolerant of thoughts other than their own," says Hamid.

In an email to The Telegraph from Finland, where he has been living since 2012, Edamaruku says, "Violence by small but organised groups is new." He tells you how his father, Joseph Edamaruku, wrote an "extremely critical" book on the Quran in 1981 and before that, a book questioning the fundamentals of Christianity. "He didn't face any threats," he adds.

Article 51A of the Constitution decrees that it shall be the duty of every citizen "to develop the scientific temper, humanism and the spirit of enquiry and reform". But the way things are, there is a sense that violence against atheists and rationalists is here to stay and even prosper.

"The Constitution guarantees that we can follow or not follow any religion but the slaves of religion simply can't tolerate us. Not because we are wrong but because we question it [religion] and its followers can't come up with answers," says Prabir Ghosh, general secretary of the Science and Rationalists' Association of India, Calcutta.

The 2011 Census revealed that 2.9 million people in India didn't wish to state that they belonged to any religion. And while it is only 0.2 per cent of India's population, it is a jump from the 2001 figure of 7 lakh.

According to Nehru Dass, the Coimbatore head of Dravidar Viduthalai Kazhagam - a rationalist group Farook was part of - atheists fear for their lives. He says, "How can they feel confident when political parties don't condemn the religious radicals? The Dravida Munnetra Kazhagam, which is supposed to be anti-religious, has also been silent on Farook's killing." And that goes for other traditionally atheistic parties in Tamil Nadu too.

Edamaruku blames it on the Centre's studied silence. "If the Prime Minister were to say that critical inquiry, scientific temper and rationalism are part of our ethos, and attacks on rationalists will not be tolerated, it would be good beginning."

Well, a rationalist should know.