It has taken nearly 50 years of trying, but the Presbyterians next week are going to heal the split that divided them 122 years ago.

If all goes according to plan, the United Presbyterian Church in the U.S.A.--the Northern church--and the Presbyterian Church in the U.S.--the Southern church--will merge on Friday afternoon in Atlanta and become simply the Presbyterian Church (U.S.A.)

The long-planned-for Presbyterian reunion will put back together the major stream of Calvinism in this country, which was divided by the controversy over slavery and the Civil War. But the architects of the reunion see the merger as a sign of hope for the future rather than just patching up the past.

"If we are able to seize the moment, I look for this event to turn the churches around," said the Rev. J. Randolph Taylor of Charlotte, N.C., who has been actively involved in reunion negotiations since l969 and is widely considered the front runner of three candidates for moderator--the top elective post--in the new church.

"The churches are more at one on this issue merger than they've been in 25 years," Taylor continued. That unanimity and enthusiasm will provide the new church with "the chance to get beyond the old polarities of evangelism and social action that have stymied us in the past," said Taylor, who was pastor of Washington's Church of the Pilgrims from 1956 to 1967.

The negotiations, planning and adjustments involved in establishing the merged church will force Presbyterians, Taylor said, "to rethink our ministry to the nation and the world. If we are able, by the grace of God, to rise to the occasion, it will make a great deal of difference."

The great national upheaval of a century and a quarter ago, which produced the Civil War, split Baptist and Methodist, as well as Presbyterian Churches, along North-South lines. Methodists got back together in 1939, although retaining a racially separate jurisdiction until a dozen years ago. Baptists have drifted so far apart theologically and culturally that reunion has not been seriously considered.

With the Presbyterians, it was not slavery as such but the issue of loyalty to the Union which precipitated the schism. The Civil War already had begun when at the May 1861 Presbyterian Assembly, the Rev. Gardiner Spring introduced a resolution calling on all Presbyterians, North and South, to pledge support to the federal government.

In a foretaste of the controversy over social action that has persisted in some quarters to this day, 57 delegates protested the action as not spiritual and thus not the province of the church. But the Spring resolution was adopted, and in the subsequent summer, as the fratricidal war progressed, 47 regional presbyteries withdrew from the General Assembly.

Subsequently, the Presbyterian Church in the Confederate States was formed, and the two streams of Presbyterianism went their separate ways.

The Rev. Dr. John Mulder, church historian and president of the Louisville Theological Seminary, operated jointly by northern and southern Presbyterians, dates the real beginning of efforts at reunion in 1937, when the southern church created a formal committee to explore reunion.

A plan for reunion was hammered out and tentatively approved by both churches by 1954. But just before final action was slated, the Supreme Court issued its school desegregation decision. "Race, as well as theological and other differences, decisively shaped the voting in the PCUS southern church presbyteries, and the plan was defeated," Mulder said.

With merger foiled at the national level, regional organizations of the two denominations began in 1969 to form union presbyteries in border states, from Texas to Washington, D.C. Seventeen union presbyteries now exist.

The Rev. Edward White, executive of the National Capital Union Presbytery, which celebrated its 10th anniversary last year, said, "It's pretty clear that it the union presbytery has been a positive experience. It has generated new energy for ministry and mission."

White cited statistics of new churches established, greatly increased participation by lay people in church affairs, and the multiplication of leadership opportunities opened to women. Creation of the union presbytery, he said, has been "synergistic, releasing new energy, new participation. It's not just been the gluing of two presbyteries together to make one bigger one, but a lot of things have shot forward to suggest that in this case, one plus one makes three."

The Rev. Jack McClendon of the historic New York Avenue Presbyerian Church recalls the merger of the presbyteries 11 years ago as creating "a good feeling. There was no feeling that this was not the right thing to do."

For the first four or five years, he said, the presbytery carefully alternated between representatives of the two parent denominations in election of a moderator "but we're over that now. Now I have to stop to think whether a particular church is North or South" in background, he said.

Despite their 122 years of separation, northern and southern Presbyterians have a lot in common. From their common spiritual ancestor, John Calvin, to their shared support and leadership in the World and National Council of Churches, they are more alike than different.

Like many mainline Protestant denominations in recent years, both are losing members. Many hope the merger may generate the kind of enthusiasm at every level to turn that around.

United Presbyterian churchman Frank Heinze has suggested tongue in cheek that through the merger, each church, by adding the other church's membership to its own, will automatically increase its membership dramatically. Under this system the UPUSA will gain 823,143 members, he points out, and the PCUS 2,387,882 members.

Though the northern church tends to be somewhat more liberal than its southern cousin, both have battled with conservatives in their midst. In fact, one reason the merger proposal received such overwhelming support in the PCUS was the defection nearly 10 years ago of a group of conservative churches who pulled out and formed the Presbyterian Church in America.

The Atlanta assembly will take the necessary actions to make the merger legal and elect a moderator. A joint committee, with 24 representatives from each of the parent bodies, has been named to work at sorting out the details of meshing two fully functioning bureaucracies. The committee also will debate such questions as whether the headquarters of the merged church will be the southern church's Atlanta offices, the New York location of the northern church, or a neutral location.

There also will be committees to make sure that affirmative action programs for both racial minorities and women, which the northern church has to some extent pioneered, are not lost in the merged church.