The Vatican secretary of state urged Iraqi Christians who have endured years of Islamic extremist persecution to forgive, reconcile and rebuild their broken communities as he celebrated Mass Friday in Iraq's largest Christian town.

Cardinal Pietro Parolin has spent the Christmas holidays in Iraq in a show of Pope Francis' solidarity with the region's Christian minorities, many of whom were forced to flee their homes during the years of extremist violence.

Parolin celebrated Mass in the Altahera Syro-Catholic Cathedral in Qaraqosh, which was overtaken by the Islamic State group in 2014. In his homily, Parolin praised those who had refused to renounce their faith and instead resisted and went into exile.

"In God's saving plan, your sacrifices will not be without fruit, as fruitful as the witness of so many martyrs who, from the first centuries of Christianity, bathed this land with their blood and lived their faith heroically to the end," he said.

He urged those families who are now returning not to dwell on revenge but to instead forgive those who wronged them, reconcile and rebuild. Many of the Christian communities of northern Iraq are some of the oldest of the faith, where dialects of Aramaic — the language of Jesus — are still spoken.

"May the pain and violence you have endured never turn into bitterness, and may the heavy yoke of hatred never fall on your shoulders," Parolin told the Iraqi religious leaders and faithful in the cathedral. "Forgiveness is the basis of reconciliation."

In the two years that the Islamic State group held Qaraqosh, militants burned down its churches, destroyed its religious altars and statues, and forced residents to convert or flee. By the time Iraqi forces retook it in 2016, Qaraqosh was practically deserted. Hundreds of families have since returned, and the global Christian community has donated generously to rebuild the churches.

There aren't reliable census figures to go by, but by all accounts Qaraqosh was once Iraq's largest Christian town with a population of around 50,000 inhabitants.

But Iraq's Christian numbers have been dwindling since the 2003 U.S. invasion, after which Christians of northern Iraq endured attacks by fundamentalist Islamic groups — foremost among them Al-Qaida.

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AP reporter Salar Salim contributed from Irbil, Iraq.