VATICAN CITY (RNS)-- Pope John Paul II welcomed the leader of the world's 250 million Orthodox Christians to the Vatican on Tuesday (June 29) with an appeal for a "leap forward" in relations between the long-divided Catholic and Orthodox churches. Ecumenical Patriarch Bartholomew I came to Rome at the pope's invitation to mark the 40th anniversary of the historic meeting between Pope Paul VI and Patriarch Atenagora I in Jerusalem that opened the way to dialogue between the churches separated by the "great schism" of 1054. "The memory of that meeting favors a leap forward in dialogue and in the strengthening of mutual fraternal relations," John Paul told Bartholomew at a private audience Tuesday morning. He asked specifically for the urgent resumption of theological dialogue by a joint Catholic-Orthodox commission. The pope acknowledged, however, that the memory of such events as the 13th century sack of Constantinople during the Fourth Crusade still impeded unity. Bartholomew is headquartered in Istanbul (formerly Constantinople) and is considered the first among equals of Orthodox hierarchs. Bartholomew agreed that dialogue "has fluctuations because of the difficulties accumulated by the history of the long division." But he called it the "responsibility and the duty" of both churches to seek unity without losing hope. The white-bearded patriarch later joined the pope in the early evening at a Mass in St. Peter's Square attended by some 15,000 pilgrims to celebrate the Solemnity of the Apostles Sts. Peter and Paul. Bartholomew, wearing a purple stole embroidered in gold with biblical scenes, sat with John Paul during part of the Mass, but separately during Communion. Together they recited the Nicene-Constantinopolitan Creed used by Byzantine churches, and each delivered a homily in Italian. A Greek Orthodox choir from Athens sang and the Gospel was read in Latin and Greek. Both the pope and the patriarch called unity a gift of God. "Your holiness," Bartholomew said, "we dream with joy of the day in which all the obstacles to full communion will be removed, and we pray continually that that day may not be long in coming." John Paul reiterated the "irrevocable" commitment of the Catholic Church to dialogue. "I want today to express the wish that all Christians intensify, each in his own way, the efforts to speed the day in which the desire of the Lord `that they may be one' will be fully realized," he said. The ailing 84-year-old pope nodded in agreement as he listened to Bartholomew, 64, and applauded at the end of the homily. When it was his turn, he spoke relatively clearly but with obvious effort. During the Mass John Paul bestowed the pallium, a band of wool symbolizing the bond of union with the pope, on 44 new archbishops, including Cardinal Justin Rigali of Philadelphia and Archbishops Sean O'Malley of Boston, Henry Mansell of Hartford, Conn., Raymond Burke of St. Louis, Mo., and Raymond Roussin of Vancouver, Canada. The visit was the third that Bartholomew has made to Rome since he was elected the 270th archbishop of Constantinople and ecumenical patriarch in 1991. He led an Orthodox delegation that traditionally attends the feast of the patrons of the Roman Catholic Church. A Vatican delegation traditionally visits Fanar in Turkey for the feast of the Orthodox patron St. Andrew on Nov. 30. Earlier this month, John Paul backed away from plans to establish a Catholic Patriarchate in predominantly Orthodox Ukraine after Bartholomew warned it could mean the end of dialogue. John Paul recalled the "blessed meeting" between Paul and Atenagora in 1964, but said he knew that the memory of "sad events of past history" weighed on efforts to end the Catholic-Orthodox schism. "In particular," he said, "in these circumstances we cannot forget what happened in the month of April in the year 1204. An army, which left to recover the Holy Land for Christianity, headed toward Constantinople to take and sack it, spilling the blood of brothers in faith." During a visit to Athens in May 2001, the pope offered a formal apology for this and other "deep wounds" that Roman Catholics have caused to "their Orthodox brothers and sisters" over the last millennium. Bartholomew said in April that he gave his "pardon" to Catholics for the sack of Constantinople. Asking for the resumption of dialogue by the mixed commission of theologians, which he described as "an important instrument," John Paul said, "I am convinced, in fact, of much urgency." "It is my will and that of my collaborators to avail ourselves of every means to feed the spirit of reciprocal welcome and comprehension in faith to the Gospel and to the common apostolic traditions," the pope said.