By James Reynolds
BBC News, Beijing
Many of China's Christians attend underground churches
At an underground church service in China, you pray as quickly as you can - and hope the police do not come running in.
At the end of an alleyway in the north of Beijing, 40 Chinese Christians gather in a small classroom. At the beginning of the service, they bow their heads and pray.
Their priest, Zhang Minxuan, stands in front of them. Twenty years ago he was a barber with no interest in religion. Then he got into trouble with the Communist Party and was jailed. After that he became a Christian.
Since then he has led an underground church and been detained a dozen times.
"One day, God will bring our church out of the darkness and into the light," he tells his followers in the classroom. Their eyes shine back at him.
"I will pray for the government no matter how much they persecute me," Mr Zhang says.
"In the end I believe that God will convert them. I will never give up my relationship with God - no matter what happens."
Underground Christians make the Chinese Communist Party nervous.
There are millions of them in this country. They worship wherever they can - often in private homes.
They do not want to be controlled by Beijing, so they refuse to sign up to the state-sanctioned church.
The party is wary of any organisation that does not pledge its loyalty to the state.
At his home in Beijing, Cai Zhuohua reads from the Old Testament.
Amity Printing Company produces about 9,000 bibles every day
In his sitting room, next to an old television set, there is a stack of bibles.
Mr Cai is another leader in China's underground Christian movement.
He is too nervous to allow us to meet his congregation - in case the police identify them from our reports.
Cai Zhuohua has been a Christian since he was a teenager.
A few years ago he had 10,000 bibles printed and delivered to fellow underground Christians. For this, the Communist Party jailed him for three years.
"I need to spread Christianity," he says, "and I need to print the Bible and distribute it to fellow believers - but I'm stopped from doing this."
So that makes what we find in the southern city of Nanjing quite a surprise.
China has its own thriving bible makers - the Amity Printing Company.
Every day the firm prints off around 9,000 bibles. But the factory is only allowed to supply bibles to the official state-approved church - not to the underground church.
The pages coming out of the presses do not seem to have much of an effect on the workers.
"I haven't read the Bible and I don't believe in Christianity," says Zhang Guohong, who's been working at the factory for 14 years.
"I have flipped through the book, but I am here to work. There is no time for me to read it."
Amity printed its first Chinese bible in 1987. Since then the company has been getting bigger and bigger.
In February 2008, Amity will move to a new site which will be able to make a million bibles a month. That may make it the world's largest bible factory.
That is quite something for the godless, Communist state.
"Perhaps it's God's humour," says Peter Dean, Amity's production advisor, "but we are printing millions of bibles here.
"We have printed 41 million bibles for the churches in China, they are distributed out through this gate, and into the networks of churches in China."
The state-approved churches are also popular
Some of the bibles end up at the Xishiku Catholic Church in Beijing.
This church is part of China's official, state-sanctioned religious establishment.
In the Catholic church, the bishops are chosen by Beijing, not the Vatican.
Everyone here answers to the Communist Party - no one has to hide or worry about getting arrested.
On Sundays hundreds of worshippers come to celebrate early morning mass. Three services are held - there are no spare seats at any of them.
This is the kind of official Christianity that the Chinese government tolerates.
The rule is simple: if you are loyal to the Communist Party, you can pray and you can worship as much as you like.
The government wants its Christians in the state-approved church where it can see them and control them.
But Christianity is growing beyond its control. One day soon, Christians may even outnumber Communists.