For the first time in four years, Christians celebrated Christmas in Mosul, Iraq's second largest city, and vowed to "rebuild the city's churches" that were destroyed by ISIS to encourage more believers to return.

On Christmas Eve, carols were sung in Saint Paul cathedral -- the only functioning church in Mosul -- for the first time since Islamic State militants took over the city in 2014, forcing the entire Christian population to flee.

The service, attended by Muslim and Christian worshippers, was led by the Patriarch of Iraq's Chaldean Catholic Church, Louis Raphael Sako, who issued a message of unity, ABC News reports.

"My message is to our brothers the Muslims before the Christians," he said. "I ask them to change their way of thinking, you should know Christianity better.

He expressed hope that Christians who fled persecution in Mosul -- about 200,000 people -- would be able to return to their homes now Islamic State militants had been driven out.

"They are not going back because their houses are destroyed or burnt and the church is restoring all of the houses," Patriarch Sako said.

He also urged congregants to pray for "peace and stability in Mosul, Iraq and the world."

"With this mass, we're sending a message of peace and love, because Christ is the messenger of peace," he said.

When ISIS overtook Mosul in 2014, the group issued an ultimatum to citizens: convert, pay taxes, leave or die. In an attempt to erase the city's Christian heritage, the group destroyed countless Christian symbols, structures, and churches.

Iraqi forces finally expelled the jihadist group from Mosul in July after months of fighting, but Christians are yet to rebuild their lives and their homes. A recent report from RT estimates that 90 percent of Mosul was destroyed or significantly damaged during the nine-month siege.

So far, between 70 and 80 Christian families have returned, according to the MailOnline.

Still, congregation member Hossam Abud, who returned this month from exile in Iraqi Kurdistan, told the outlet the service is a "sign that life is returning to Mosul."

"We must rebuild the city's churches that were destroyed to encourage the return of Christians," he said.

Farqad Malko, a Christian woman in the congregation, said the service was a message to the terrorist group.

"With this celebration, we tell them that residents of Mosul are all brothers, whatever their religion or ethnicity, and despite all the damage and suffering," she said.

Mina Ramez, a 20-year-old Christian who returned to Mosul with her family two months ago, added, "This is our land, these are our homes, and we will do everything we can with our brothers of all religions to rebuild it. We will never abandon the land of our birth."