When Pope John Paul II joins the archbishop of Canterbury before the altar of the ancient Canterbury Cathedral, as he is scheduled to do today, it will be one of the poignant moments in Christian history.

The Rt. Rev. Robert Runcie, the present archbishop, has called the Roman pontiff's acceptance of the invitation to the service "unprecedented in the history of the church in the West."

But what contribution the papal visit will make in healing the breach News Analysis News Analysis between the Roman Catholic Church and the Church of England, which split from it 448 years ago, remains for history to judge.

For the past two decades, the prevailing spirit of mainstream Christianity has been ecumenism, an effort to concentrate on what the diverse branches of Christendom have in common and, where possible, bridge differences. Within that climate, Anglican-Catholic relations had progressed so amicably a little less than a decade ago, that one of Runcie's predecessors predicted reunion of the two churches by the end of the century.

But the present pope is generally perceived to exhibit less zeal for ecumenism than either John XXIII or Paul VI, under whom the most impressive gains were made. In addition, the worldwide ecumenical movement today has lost some of its momentum.

Roman Catholicism ceased to be the official religion of England in 1534, after Pope Clement VII gained the enmity of Henry VIII, first by refusing to annul his marriage to the childless Catherine of Aragon, and then by excommunicating Henry when the defiant monarch discarded Catherine for Anne Boleyn.

Henry countered with the Act of Supremacy, which cut Britain's ties with Rome and made the British monarch the head of the Church of England.

The king's action came during a period of religious ferment in Europe: the time of Martin Luther with his 95 theses in Germany and the rising influence of Ulrich Zwingli and John Calvin in Switzerland.

For centuries, Roman Catholic-Anglican relations were characterized by mutual hostility. It was not until 1829 that the Catholic Emancipation Act lifted the last of the civil disabilities that Henry had levied against Catholics in his efforts to stamp out the faith. A Roman Catholic bishop was not reestablished in Britain until 1850.

But with the burgeoning ecumenical mood of the mid-20th century, helped greatly by the 1958 election of Pope John XXIII, the climate of mutual anathema abruptly changed to a mutual yearning for unity.

In 1960, the 99th archbishop of Canterbury, the Rt. Rev. Geoffery Fisher, capped a distinguished and often controversial career by what was in its day an audacious move--an official visit to the Vatican and Pope John.

Six years later, Fisher's successor, the Rt. Rev. Arthur Michael Ramsey, paid a call on Pope Paul VI. Their conversations led to an agreement to establish a scholarly commission to study the question of Anglican-Roman unity.

This body, the Anglican-Roman Catholic International Commission, completed its final report late last year, the fruit of 12 years of study and dialogue between theologians and scholars from both churches.

Representatives of the two churches said they had found essential agreement on such key questions as the meaning of the eucharist, or holy communion, on the doctrine of the ministry and on the role of authority in the church.

While acknowledging that some in their church would find it hard to accept, Anglican members of the commission even agreed on the need for a "universal primate" in a reunited church. And while they purposely avoided the word "pope," commission members noted that the Roman pontiff was the most logical candidate for the post.

The report, officially released in March, is now before both churches for study. The Vatican's Congregation for the Doctrine of the Faith, formerly known as the Holy Office, has called the report a useful basis for further steps toward reconciliation, but said "it does not yet constitute a substantial and explicit agreement on some essential elements of Catholic faith."

The congregation listed as doctrinal difficulties the eucharist, the theological role of priesthood, the ordination of women and papal infallibility.

As the theologians were studying unity, there were unsettling developments that pulled in the opposite direction. Anglicans, especially those in the Episcopal Church in this country, were affronted by the arrangements developed by the Vatican two years ago to accept into the Roman church dissident Episcopal priests who left their own church in a dispute over ordaining women to the priesthood and liturgical reform.

The battle over ordination of women to the priesthood continues in both communions; it was not dealt with in the ARCIC report. Five regional bodies of Anglicanism, including the churches in this country and Canada, currently ordain women. In other countries where the issue has come up in Anglicanism, particularly in Britain itself, the premise that ordaining women would be an impediment to Anglican-Catholic unity is always raised by opponents of ordaining women.

The question of permitting priests to marry--Anglicans do, Catholics don't--is less of an issue in unity talks, since some Eastern Rite units of the Roman Church permit priests to marry.

Much of the anti-Catholicism that exists in Britain is focused on the pope and has found a new focus in John Paul's visit. Particularly tight security is planned for the visit to Canterbury Cathedral, an event that has particularly enraged anti-Catholic extremists. In an ugly incident in March, Runcie, who has become well known as an ecumenist, was shouted down by anti-Catholic demonstrators in a Liverpool parish.

Pictures of John Paul and Runcie--Rome and Canterbury--joined in prayer, and with Britain's future defender of the faith, Prince Charles, looking on, will be a powerful symbol. Whether it becomes a symbol for unity or the occasion for more discord is up to history. the Doctrine of the Faith, formerly known as the Holy Office, has called the report a useful basis for further steps toward reconciliation, but said "it does not yet constitute a substantial and explicit agreement on some essential elements of Catholic faith."

The congregation listed as doctrinal difficulties the eucharist, the theological role of priesthood, the ordination of women and papal infallibility.

As the theologians were studying unity, there were unsettling developments that pulled in the opposite direction. Anglicans, especially those in the Episcopal Church in this country, were affronted by the arrangements developed by the Vatican two years ago to accept into the Roman church dissident Episcopal priests who left their own church in a dispute over ordaining women to the priesthood and liturgical reform.

The battle over ordination of women to the priesthood continues in both communions; it was not dealt with in the ARCIC report. Five regional bodies of Anglicanism, including the churches in this country and Canada, currently ordain women. In other countries where the issue has come up in Anglicanism, particularly in Britain itself, the premise that ordaining women would be an impediment to Anglican-Catholic unity is always raised by opponents of ordaining women.

The question of permitting priests to marry--Anglicans do, Catholics don't--is less of an issue in unity talks, since some Eastern Rite units of the Roman Church permit priests to marry.

Much of the anti-Catholicism that exists in Britain is focused on the pope and has found a new focus in John Paul's visit. Particularly tight security is planned for the visit to Canterbury Cathedral, an event that has particularly enraged anti-Catholic extremists. In an ugly incident in March, Runcie, who has become well known as an ecumenist, was shouted down by anti-Catholic demonstrators in a Liverpool parish.

Pictures of John Paul and Runcie--Rome and Canterbury--joined in prayer, and with Britain's future defender of the faith, Prince Charles, looking on, will be a powerful symbol. Whether it becomes a symbol for unity or the occasion for more discord is up to history.