Despite its four ministers, two buildings and eight years of success, the venture still is considered an experiment in many church quarters. But to the 1,300 members of the United Christian Parish in Reston, their unusual community long since has outgrown the experimental stage and now is solidly established.

The United Christian Parish is an ecumenical church whose members, from several Protestant traditions, have set aside their theological differences to worship as a united Christian body.

In overcoming the tensions and growing pains of melding members of five denominations into a single congregation, UCP, regarded as a phenomenon by some church leaders, not only has flourished but also has gained fame nationally as a model ecumenical church.

The reach of that reputation was brought home personally to the Rev. Douglas Ibach, a UCP co-pastor, during a visit to Seattle last year when "my wife and I were walking down the street under an umbrella in the pouring rain . . . [and] we heard the couple walking in front of us talking about a large ecumenical church they'd heard about in Reston, Va." Ibach, a true believer in the church's concept, rushed to give the couple a firsthand account.

His appreciation of the parish is shared by numerous Protestant church figures, including the Rev. Robert Huston, a staff member of the United Methodist General Commission on Christian Unity and Interreligious Concerns, who says, "They are one of the best examples of responsible Christian unity on a local basis."

This month the United Council of Churches awarded UCP its annual Ecumenical Service Recognition Award for the church's "creative ecumenical approach to new church development. . . ."

Most UCP members come from the United Methodist Church, the United Presbyterian Church in the U.S., the Presbyterian Church of the U.S., the United Church of Christ or the Disciples of Christ. Upon joining UCP, they automatically belong to the four other denominations that make up the church.

All five denominations recognize the credentials of the church's ministers, who come from those Protestant traditions.

With its cushion of eight years in operation, UCP, except for its intricate administration, operates much like any other church. Members worship together, have one parish board and one budget, partake of youth and religious education programs, contribute to missionary and other charitable funds and participate in community outreach services.

Ministers take turns leading services at three locations. UCP, which started with 500 members, already has outgrown its original church and now operates from two churches and rented space at a local school. A third church building is planned for 1984 and a fourth is expected in later years, according to Ibach, one of the church's three full-time copastors.

"I think we must be doing something right," said Bill Jackson, a founding member and secretary on the parish board. "When people leave here they tend to take a long time in finding another church because they're looking for another ecumenical church," he said. "But they write and tell us they can't find a church they like as much."

One the road to success, local denominational leaders, clergy and members had to wrestle with such issues as how to satisfy five traditions in liturgy, sacraments and church government; discipline of lay members or clergy; housing of ministers; financial responsibilities, and the respective roles of the three copastors, each of whom has equal say.

Much were addressed by a covenant spelling out procedures for government and the sacraments, the rest by mutual cooperation. For example, UCP offers three slightly different worship styles in the three different meeting places; three forms of baptism -- infant, adult by immersion and adult by sprinkling; and a choice of wine or grape juice with communion. Communion alternately is distributed at the altar rail and in the seats.

Members even have a choice of ministerial garb; one wears robes and the others wear suits while conducting services.

Money matters are a little more complicated. All offerings go into a common fund, according to Ibach, with money allotted from that for local outreach, charities associated with the five denominations and denominational support.

Making it all work "just takes a little more creativity," said Ed Hansen, parish moderator and a founding member. "But I like it fine. We end up getting to share the heritage of each of the denominations."

One thing UCP sponsors did not do was compromise, said Ibach. "If we could have compromised and appealed to the lowest common denominator we could have avioded a lot of tension . . . fulfilling all the governmental rules of each."

Cooperation has played its part but UCP's success has reuslted in no small measure from being in a growing, planned community that has protected it from competition.

Because the parish's five denominations agreed during Reston's early planning stages to refrain from forming separate congregatio;ns, residents wishing to worship in any of those traditions had to join UCP or drive to a neighboring community.

Those factors plus the parish's ability to deal with its growing pains -- a period that Hansen describes as difficult but eminently worth it -- have built a congregation free to choose from five denominations' offerings in liturgy, religious education, speakers, retreats, summer camps and enrichment programs.

"We [in leadership positions] have to attend five times as many meetings, report to five different congregations and please five times as many backgrounds," said Jackson. "ybut our opportunities are quadruple."

Some, like Howard Barnard, find UCP unusually warm and friendly. "I looked all over Northern Virginia for a church where I'd feel welcome," Barnard said. "The only one I found was United Christian Parish. One church we visited four Sundays in a row and even went to Sunday school and nobody even said hello."

Barnard was looking for a Presbyterian, not an ecumenical church, he said, and was pleased that at UCP "I still feel like a Presbyterian.

"It always puzzled me that so many Protestant denominations are supposed to be preaching the same thing but . . . somewhere somebody got mad at somebody and they split. It doesn't make any sense to me," Jackson said.

To him and other founding members, an ecumenical church remains a simple, logical and economical approach to community worship, although they have discovered that a large ecumenical church is not significantly cheaper to operate than several independent churches. (Some building costs are avoided, but with three locations to run, there is unavoidable duplication of office staff and clergy at UCP, Ibac said.)

Despite their pride and satisfaction in the parish's success, there is a sense, as one minister put it, of seeming to be an anachronism at a time when many mainline denominations have turned their attention away from the 1960s ideal of ecumenism to more pressing issues such as declining membership.

But parish leaders and members believe that the wheel will turn again, and they are joined by David Taylor, ecumenical officer of the Presbyterian Church in the U.S., who predicts a rise in ecumenical churches.

"Frankly, having competing churches across the street from each other doesn't make good religious, philosophical or financial sense," Taylor said. "Frequently communities either cannot support two or three different churches or are refusing to do so, especially in new communities.

Our faith teaches us that there is only one church, that there is no such thing as churches in the plural."