It comes on the final day of the Pope's historic visit to Iraq.
Pope Francis prayed Sunday for "victims of war" outside a ruined church in Iraq's Mosul, where the Islamic State group ravaged one of the world's oldest Christian communities until the jihadists' defeat three years ago.
With the partially collapsed walls of the centuries-old Al-Tahera (Immaculate Conception) Church behind him, Pope Francis made a plea for Christians in Iraq and the Middle East to stay in their homelands.
The 84-year-old pontiff said the "tragic" exodus of Christians from Iraq and the wider region "does incalculable harm not just to the individuals and communities concerned, but also to the society they leave behind".
The IS onslaught forced hundreds of thousands of Christians in northern Iraq's Nineveh province to flee. Iraq's Christian population has shrunk to fewer than 400,000 from around 1.5 million before the US-led invasion of 2003.
The faithful had gathered on Sunday in the courtyard of the Al-Tahera Church, whose roof collapsed during fighting against IS in 2017. It is one of the oldest of at least 14 churches in Nineveh province that were destroyed by IS.
Boutros Chito, a Catholic priest in Mosul, said the pope's visit could change the way people think about his city, the ancient centre of which still lies in ruins.
"Pope Francis will announce to the whole world that we are the people of peace, a civilisation of love," Chito told AFP.
The pontiff also hailed the Al-Nouri mosque and its famed al-Hadba minaret, which were destroyed in the fighting against IS, and the Church of Our Lady of the Hour which, he said, "has reminded passersby that life is short and that time is precious".
The heaviest deployment of security forces yet has been mobilised to protect the Pope on what is perhaps the riskiest day of his historic trip to Iraq, where state forces are still hunting IS sleeper cells.
Pope Francis's trip to Iraq as a "pilgrim of peace" aims to reassure the country's dwindling Christian community and to expand his dialogue with other religions.
On Saturday, the leader of the world's 1.3 billion Catholics met Iraq's top Shiite Muslim cleric, the reclusive Grand Ayatollah Ali Sistani, who agreed that Iraq's Christians should be able to live in "peace".
"We believers cannot be silent when terrorism abuses religion," he said at an interfaith service in the ancient site of Ur later that morning.
Watching from afar as IS swept across Nineveh in 2014, Pope Francis said at the time he was ready to come and meet the displaced and other victims of war in a show of solidarity.
Seven years later, he is visiting both Mosul and Qaraqosh, one of Iraq's oldest Christian towns whose residents still speak Syriac, a dialect of Aramaic, the language spoken by Jesus.