Resolving one of the major doctrinal disputes in Christendom, five Oriental Orthodox churches that broke from Rome 15 centuries ago have agreed in principle to seek reunification with the Roman Catholic Church.

The five have a combined membership of 17 million, about 1 million of them in the United States.

While serious nondoctrinal obstacles remain to be resolved, theologians of the five churches and the Roman church meeting at Vienna, Austria, this week were reported to have settled the doctrinal controversy that led to the first major schism in the year 451.

A conclave of the world's christian leaders called that year to resolve a dispute over the nature of Christ adopted a doctrinal formulation that the five Oriental churches refused to accept.

Since then the five Oriental churches - the Coptic Orthodox Church of Egypt, the Syrian Orthodox Church of Antoioch, the Armeanian Apostolic Church, the Ethiopian Orthodox Church and the Syrian Orthodox Church of Indian - have remained a distinctly separate branch of Christianity that viewed Christ's nature as only divine.

The 451 conclave held at Chalcedon in Asia Minor (present-day Turkey), overwhelmingly adopted the formula of two natures in Christ - the human as well as the divine - united as a conceptual entity.

According to sources reached by telephone in Rome and Vienna, the settlement of doctrinal disputes and the general agreement to seek reunification are not likely to lead to any immediate steps to incorporate the five small churches into Christendom's largest organized branch.

They said the key obstacle is the pope's role as head of the Roman Catholic Church.

The role of the pope or the question of church authority, led to the second major schism in Christendom, between Rome and Constantiople, in the 11th century. Although doctrinal differences were minor, the split over the leadership between the Roman Catholic Church and the Orthodox Churches of Eastern Europe has continued since.

The tentative agreements reached at Vienna are viewed by theologians as important since they heal a doctrinal separation more than 1,500 years old and provide an added impetus to the ecumenical spirit generated by Pope John XXIII.

Apart from "setting an eccumenial pattern," according to American Catholic theologians, the settlement of the 1,500 year-old dispute demonstrated the ability of both sides to take a broader view in overcoming the terms of the original arguments.

According to the Rev. Avery Dullers of Catholic University, "the issues which originally separated the five Oriental churches are not crucial issues anymore. At stake (in the future reunificaion effort) are subsequent developments in the West to which these churches were not a part."

The Vienna talks, which lasted for a week, cap more than 10 years of discussions sponsored by Cardinal Franz Keonig of Vienna. A similar dialogue opened last January in the United States between Roman Catholic theologians and those of the five Oriental Orthodox churches here.

Political aspects of the possible reunification are hard to assess. The five Oriental churches flourished in Asia Minor until the Turkish conquest of the region. The Islamic Turks practically wiped them out in the entire region except Ethiopia.

The Ethiopian revolution in 1974 dealt a severe blow to the Orthodox Church of Ethiopia by confiscating its lands and stipping it of its privileges. The new Marxist rulers of Ethiopia are engaged in a continuous antoreligin campaign.

The seat of the Armenian Church is in Soviet Armenia. There, too, the authorities are trying to combat religious influences. The Armenian Aposlolic Church of America has more than half of the five churches' total American membership.

The Syrian Church and the Coptic Church of Egypts serve small Christian minorities in the two Arab countries. The Syrian Church of India involves about one million Christians, most of them in the state of Kerala.