A man on trial in southern Russia faces a possible one-year prison sentence for having written "there is no God" during an internet exchange.
Viktor Krasnov was reported to police by two young men who objected to his language in the dispute, on the Russian social network VKontakte in 2014.
He was charged in Stavropol for having "insulted the feelings of worshippers".
Such "insults" were outlawed in 2013 after the Pussy Riot case, in which two punk performers were jailed.
There was international condemnation in 2012 when Nadezhda Tolokonnikova and Maria Alyokhina of Pussy Riot were sent to labour camps for having performed a crude protest song in Moscow's main cathedral.
VKontakte is a Facebook-style networking site popular among Russians. In the exchange Mr Krasnov also dismissed the Bible as a "collection of Jewish fairy tales".
Linguistic experts supported the plaintiffs' allegation that Mr Krasnov's remarks were "insulting to worshippers".
The relevant Russian law provides several alternatives to a one-year prison term in such cases, including a fine of up to 300,000 roubles (£2,900; $4,083) or up to 240 hours' forced labour.