The Episcopal Church on Tuesday became the largest US church to authorize blessings for same-sex couples. The historic step marks a victory for activists, observers say, but stands to hamper the church’s already-strained international relationships.

At the General Convention in Indianapolis, 78 percent of laity and 76 percent of clergy in the House of Deputies voted to support the new liturgy Tuesday. This followed the lead of the church’s House of Bishops, which gave its approval Monday.

The vote equips priests and congregations, starting Dec. 2, to use an official church resource to bless lifelong commitments made by same-sex couples. Usage is restricted: Congregations and priests must first secure their bishop’s permission to use it, and it may not be used in civil marriage ceremonies. But proponents were jubilant nonetheless.

“For the church to say, ‘This is an active part of our life in ministry, and we support this,’ is an extraordinarily important step,” said the Very Rev. Katherine Hancock Ragsdale, president of Episcopal Divinity School in Cambridge, Mass. “It shows that when this church says it welcomes all people, it means it.”

With this week’s votes, the 2 million member Episcopal Church goes where some (though not all) other Protestant denominations have hesitated to tread. Just last week, the Presbyterian Church (USA) narrowly defeated efforts to redefine marriage in its constitution to include gay couples. In May, the United Methodist Church reaffirmed its teaching that same-sex relationships are incompatible with Christian teaching. The Evangelical Lutheran Church in America, meanwhile, permits same-sex ceremonies but has not created a rite for blessing them.

Yet since 2005, the 1-million-member United Church of Christ has supported same-sex blessings.

Unlike other Protestant groups, the Episcopal Church belongs to a worldwide church that has called for a moratorium on same-sex blessings. The 80-million-member Anglican Communion includes the Episcopal Church among its 34 provinces. Some fear this week’s adoption of a same-sex liturgy will add further strain to already-frayed relationships.

“It means the Episcopal Church is now separating itself that much more from the Anglican Communion,” says Hood College historian David Hein, co-author of “The Episcopalians,” a standard history of the church. “The American Episcopal Church is trying to set itself up as a separate denomination, although they would claim that they’re not.”

The Episcopal Church has spent decades cultivating closer ties with other Christian groups, most notably Lutherans, Roman Catholics, and Orthodox churches. Authorizing a same-sex liturgy could spell trouble for these ecumenical relationships, observers say. In 2010, the Anglican Communion asked Episcopalians to resign their posts in ecumenical dialogues because their church had defied the moratorium on same-sex blessings.

But activists are nonetheless celebrating a victory, even if it brings costs.

“We are not going to be blackmailed into bigotry against our gay and lesbian brothers and sisters in order to maintain a unity that requires uniformity,” says the Rev. Susan Russell, senior associate at All Saints Church in Pasadena, Calif.

Supporters of the new liturgy don’t expect it to cause another wave of conservative departures along the lines of what the church saw after the 2003 consecration of V. Gene Robinson as the Episcopal Church’s first openly gay bishop. That row over sexuality issues helped spawn the 4-year-old, theologically conservative Anglican Church in North America, which has attracted more than 800 congregations, including hundreds from the Episcopal Church.

“The issue that’s causing the [Episcopal Church’s decline] is not the inclusion of gay and lesbian people,” Ms. Russell says. “The problem has been the fighting about it.... We’ll have great energy for proclaiming the Good News to everyone once we’ve put these inclusion wars behind us.”

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Others, however, aren’t convinced that growing consensus around same-sex issues signals a brighter future. The Episcopal Church has lost 44 percent of its members since 1965, Mr. Hein notes, even though the US population has been growing during that period. He expects decline to continue.

“They’re losing 1,000 members a week,” Hein says. “The Episcopal Church has now positioned itself way out there on the edge with the United Church of Christ, which is extremely liberal and also losing members.”