Cardinal Vincent Nichols made the comment as the ground assault to retake Mosul from Islamic State militants reaches its eighth day.
Iraqi and Kurdish forces, backed by US-led airstrikes, have reported the operation to retake the city is going quicker than expected.
Islamic State has been using suicide bombers, human shields, road mines and oil fires as well as conventional artillery to defend Iraq's second city, which still has more than a million people trapped inside.
Kurdish Peshmerga forces recently surrounded the town of Bashiqa, eight miles east of Mosul, which lies on an important supply route.
They also retook the Christian town of Bartella, nine miles east of Mosul, earlier this week.
Cardinal Nichols met with refugees in the Iraqi city of Irbil, 85km to the west of Mosul, last year.
He said: "My prayers are with the people of Mosul as efforts continue to liberate the city and surrounding areas from the tyranny of Daesh.
"It is vital that protection of civilians, respect for humanitarian conventions and support for those fleeing their homes are at the forefront of this operation.
"It will undoubtedly take time for the physical liberation to be realised.
"Only then can the hard work begin to establish the rule of law and rebuild the region's diverse social fabric of which the Christian community, many of whom I met in Irbil last year in the refugee camps offering them sanctuary from the violence, is an integral part.
"This rebuilding is a crucial step which will allow them and other refugees to return to their homes."
Mosul used to be home to hundreds of thousands of Christians and is a historic resting place for several Old Testament prophets.
Islamic State captured the city in 2014 on their way to taking almost a third of all Iraq.
They've lost significant territory since then, with Mosul being their last military bastion.
