[Catholic Archbishop of the Greek islands of Naxos, Tinos and Mykonos, Nikolaos oversees preparations for a ceremony at the port, at the city of Mytilene on the Greek island of Lesbos]By Karolina Tagaris and Philip Pullella LESBOS, Greece/VATICAN CITY (Reuters) - Pope Francis and the spiritual head of the world's Orthodox Christians will set aside centuries of dispute on Saturday when they visit a Greek island on the front line of Europe's migrant crisis to highlight the plight of refugees. Nearly a million people, many fleeing war, persecution and poverty in Syria, Iraq and Afghanistan, have risked their lives in rickety boats from Turkey, crossing to Lesbos before heading to the Greek mainland and onwards to western Europe. In a lightning trip, the pope will meet refugees along with Bartholomew, the Istanbul-based spiritual head of the world's 250 million Orthodox Christians, and Ieronymos II, head of the Greek Orthodox Church.