A missionary who was once detained in a Chinese prison recounted how he and the other prisoners expressed their worship to Jesus by whistling hymns.

Peter was detained in 2003 for helping North Korean defectors in China. He was studying Chinese in preparation to be a missionary in the country but he ended up helping North Korean refugees, according to the International Christian Concern (ICC), an organization that helps persecuted believers worldwide.

During that time, North Korea was ravished by an intense famine called the Arduous March. It left an estimated 3 million people dead because of starvation.

A defector who witnessed the famine said she saw people in the countryside who looked like they were "living dead." She also saw people falling dead on the street because of hunger, and described how "the smell of decomposing bodies was everywhere," according to the Thomson Reuters Foundation.

Many North Korean women, in an attempt to escape the famine, defected to China and sold themselves as sex slaves. When some of them got pregnant, they were sent back to North Korea, where they had forced abortions and were tortured. The plight of these North Korean women gripped Peter's heart.

He abandoned his plans to be a missionary to China and started helping North Korean refugees so they can go to South Korea. Eventually, he got caught by North Korean spies, and he was sent to a Chinese prison where he shared a 100-square foot cell with 35 men, most of whom used to be tough gang members.

To avoid fighting with them, he would smile at them and say, "Hallelujah." Yet, tough as they were, the other prisoners started asking him questions about God.

"Is God really alive?" they asked.

After answering their question, Peter's attention was called by one of the prisoners, who asked him to pray for his stomach.

"I just touched his hand, and he got cured," Peter told ICC. "The Chinese criminals were very shocked. Even I was shocked!"

After that, Peter listened to sermons daily through a radio he asked for from a gang leader. He would take down notes from the sermons and teach them to his cellmates. Soon, prisoners in other cells heard about what was happening in Peter's cell, and some of them asked the guards to be transferred there.

Peter thought of a way they could worship without alerting the prison guards, and he decided to teach them to whistle to the tune of hymns, such as the classic 'Amazing Grace.'

He said the whistling would begin in one cell, and as prisoners in other cells heard it, they would join in until the entire prison resounded with their worship-and the prison guards had no clue what was going on.

"That was an amazing time," Peter said. "I would spend one hour praying, one hour resting, one hour praying, one hour resting ... all day long."

When he was released, he continued his work of helping North Korean refugees, but this time in Vietnam. He was caught by the authorities again, and even though he begged God not to allow him to be sent to prison, he was incarcerated for a few weeks.

In the short time he spent in the Vietnamese prison, he was able to preach the gospel. One of them was baptized in his cell.

"I think preaching to the Vietnamese people in prison was God's will," he concluded.

North Korea continues to be number one in Open Doors' World Watch List of countries where Christians are most persecuted.

"North Korea is ranked as the most oppressive place in the world for Christians," Open Doors says. "Entire Christian families are imprisoned in hard labor camps, where unknown numbers die each year from torture, beatings, overexertion and starvation."

"Those who attempt to flee to South Korea through China risk execution or life imprisonment, and those who stay behind often fare no better."