To give Mrrzy a little backup here:

I don't care what most people believe - they have the right to believe any damfool thing they care to, and I can usually be polite even if someone describes to me the little spirits that look after their garden (true incident).

But there aren't a huge number of garden fairy believers. The belief is harmless to others. The big religions - Islam, Christianity, Hinduism - are far from harmless, and can and do impact people who don't believe in them, through such things as the oppression of women and minorities (all three undervalue women, Hindi caste system), the suppression of science (particularly medical science: opposition to birth control, opposition to HPV vaccine, also evolution), and restriction of freedom (especially sexual freedom: see treatment of gays in Islam).

Why should I hold my tongue and pretend to respect a belief that says some kinds of people have less value than others? Why should I hold my tongue and pretend to respect a belief that tries to hinder science? Why should I hold my tongue and respect a belief that threatens me with death if I don't conform to the rules of that belief?

Why shouldn't I vigorously oppose such beliefs and call them wrongheaded, nonsensical, mediaeval?