"Do it or die!"

That was the command Iraqi Christians said they were constantly subjected to by Islamic State (ISIS) militants during the two years when they were under ISIS rule.

ISIS militants swept across the Nineveh Plain in northern Iraq in August 2014 and told Christian residents to choose one of four options—convert to Islam, pay tax, leave, or die.

Around 120,000 of them opted to flee. But those who did not get a chance to leave faced one of the three other options.

Following their town's liberation by the Iraqi army, the remaining Christian residents revealed the various ways they were brutally tortured and threatened by ISIS militants and the humiliating and degrading acts they were forced to do, Agence France Presse (AFP) reported.

Zarifa Bakoos Daddo, 77, is one of these Christian survivors. She recalled that one time she and an elderly friend living with her were in their house in Qaraqosh when a young ISIS militant came and told them to convert to Islam.

"I told him we had our beliefs and they had theirs," she said.

Her response appeared to have angered the jihadist who then ordered her to spit on a picture of the Virgin Mary and a crucifix. "I refused but he made me. The whole time I was telling God in my heart that I did not mean any of this," Zarifa said.

The militant then tried to burn the picture of Jesus' mother—but no spark came from his lighter. "I knew God heard me because he tried to burn the picture and his lighter didn't work," Zarifa said, causing laughter to members of her family in their home in the liberated town of Arbil.

In another incident, she said one of the ISIS fighters came to their house asking for money and gold. "He poked his rifle into my ribs and said 'You have to give to us'," she said.

Zarifa handed over the $300 she had left while her friend gave her 15-carat gold.

Days after Iraqi forces liberated Qaraqosh late last month, she and her friend finally heaved a sigh of relief.

"The whole time I prayed for my people, for the town, and also for these Daesh members, that God may open their hearts," she said.

Finally reunited with her family, Zarifa said the joy she feels has already erased her suffering.

Ismail Matti, 16, is also another survivor with a gripping story to tell. He said when the ISIS jihadists arrived in his hometown of Bartalla, east of Mosul, he and his sick mother were thrown in prison just like other residents who tried to flee.

"There were Shiite people crammed in a cell next to ours. They took one, shot him in the head and dragged his body in front of us," he said.

"They told my mother the same thing would happen to me if we refused to convert. So we converted," Ismail recalled.

They were then allowed to go back to a village in Bartalla where all their "neighbours were Daesh," using an Arab acronym for ISIS. "They would come to check if I was following the Sharia (Islamic law)."

"If they found that I hadn't been to the mosque to pray, I sometimes got lashes," Ismail said.