Eric Foley is co-founder of Seoul USA, an American missionary organisation which has dropped 50,000 Bibles into North Korea by balloon this past year.
Bibles are rare in North Korea. Houses are searched by Government officials. If a Bible is found, the family is either sent to a concentration camp or executed.
Christians don't reveal their faith to their spouses until years after marriage. They can't tell their children of their faith until they are at least 15, as teachers are trained to extract such information from pupils.
There are ways, however. One grandfather gathered his family together each week to give them the same 10 pieces of advice. Later, a grandson realised he was passing down the 10 Commandments. Christians can't risk gathering together because spies are everywhere. They worship in their own homes, or walking down the road out of the earshot of others.
When Communism took over and Bibles were banned, Christians chose four pillars of Christianity to pass on to new believers - theology through the Apostles' Creed, prayer through the Lord's Prayer, ethics through the 10 Commandments and worship through the Lord's Supper.
Foley estimates there are 100,000 Christians in North Korea, about a third of them in concentration camps. They regard concentration camps as a mission field, their purpose to carry out the task God has given them regardless of cost.
Foley asked a North Korean believer how he could pray for them. "Pray for us?" the Christian asked. "We pray for you. South Korean and American churches believe challenges in the Christian faith are solved by money, freedom and politics. It's only when all you have is God you realise God is all you need."