It's almost 40% atheist. What nation beats it? I'm only counting nations with freedom of religion though, not communist ones that ban religion.
Countries with freedom of religion whose population is over 40% atheist :
The Netherlands, Estonia, Cuba, Taiwan, Slovenia(not sure about this one), and a few others.
Countries where church attendance is lower than the Czech Republic:
Korea, South
Taiwan
Ukraine
Moldova
Bulgaria
Georgia
China
Armenia
Serbia and Montenegro
Belarus
Azerbaijan
Latvia
Norway
Denmark
Finland
Estonia
Iceland
CIA World Factbook on these:
Czech Republic: 39.8% atheist (narrowly beats out Catholics for a plurality)
Netherlands: Roman Catholic 31%, Protestant 21%, Muslim 4.4%, other 3.6%, unaffiliated 40% (1998)
unaffiliated doesn't neccesarily mean atheist but granted it is slight higher than the Czech Republic
Cuba: only gives stats before Castro came to power, but like I said, I'm only talking about countries with freedom of religion
Estonia: gives a list of religions but no percentages. I looked it up on wikipedia and it says Estonia largest religious group is Evangelical Lutheran (which I am) but only a third of the country has any religious affiliation. So this one beats it I guess
Slovenia: over 70% Catholic, 4.3% atheist (lower than the US)
Taiwan: mixture of Buddhist, Confucian, and Taoist 93%, Christian 4.5%, other 2.5%