Unable to buy a train ticket, or even see a doctor at a hospital, a Chinese pastor found that even after his release from prison, he is not quite free.

The Rev. John Sanqiang Cao was arrested and sentenced to seven years in prison while coming back from a missionary trip in Burma. Now back in his hometown of Changsha in southern Hunan province, he is without any legal documentation in his country, unable to access even the most basic services without a Chinese identification.

"I told them I’m a second-(class) Chinese citizen, I cannot do this, I cannot do that," Cao in an interview with The Associated Press. "I’m released, I’m a free citizen, why should there be so many restrictions upon me?"

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Cao, who was born and raised in Changsha, had dedicated his life to spreading Christianity in China, where the religion is strictly regulated. He had studied in the U.S., married an American woman and started a family, but said he felt a calling to go back to his home country and spread the faith.

It's a risky mission. Christianity in China is allowed only in state-sponsored churches, where the ruling Communist Party decides how Scripture should be interpreted. Anything else, including clandestine "house" churches and unofficial Bible schools, is considered illegal, though it was once tolerated by local officials.

Cao was undeterred, citing the courage of Chinese Christians he had met who spent time in prison for their faith. During his years in China, he said he had set up some 50 Bible study schools all across the country.

In the years leading up to his arrest, he had started bringing Chinese missionaries to parts of northern Burma that had been impacted by the country's civil war. They focused on relief work, campaigning against drug use, and setting up schools in areas bordering China.

It was in coming back from one of these crossings that he was detained in 2017. He was sentenced to seven years on a charge of "organizing others to illegally cross the border," which is usually reserved for human traffickers.

His family and supporters had advocated for Cao's sentence to be reduced, but to no avail. Cao was a prisoner of conscience, according to the federal U.S. Commission on International Religious Freedom, which also called for his freedom.

After completing his sentence, Cao is no longer behind bars. But he is facing another major obstacle.

He said that police who came to his mother’s house in 2006 took away her "hukou" registration book, which had also included Cao.

Every child born in China is registered in the hukou, which is an identification system through which social benefits are allocated by geography. Later in life, the hukou is needed to apply for a national ID card, which is used in everything from getting a phone number to public health insurance.

According to Cao, police said they would help his mother update the hukou. It was only later that he found out in updating her registration that they removed his name.

Cao never took American citizenship because of his calling, spending his time between the two countries. He had kept his U.S. permanent residency throughout this time, though he says that’s not accepted as an ID in China.

He was traveling on his Chinese passport. Though he took note of the fact that he no longer had the hukou registration, he did not realize how serious the problem was until much later.

In prison, his Chinese passport had expired, he said, and he could not renew it.

Cao said he has been to the police station many times since his release, and had even hired a lawyer. So far, he said police had not given him a satisfactory answer as to why his records no longer exist.

A police officer at the Dingwangtai police station in Changsha, where Cao's hukou registration is supposed to be, said he did not know how to address Cao's claims. "Even if he went to prison, he should still have a hukou," he told the AP. The officer refused to give his name because he wasn't authorized to talk to media.

Cao's two adults sons were able to visit him this month, spending two weeks with their father. Cao said he wants to join them and his wife in the U.S., though it's unclear how he can do that.

"I moved from a smaller prison ... to come to a bigger prison," he said.