"Christianity has undergone one of the greatest demographic and cultural shifts in its 2,000-year history," she said.
Given the amazing diversity among Christ's followers, the worldwide spread of Christianity requires that mission be integral to unity, said the co-director of Boston University School of Theology's Center for Global Christianity & Mission.
"Contemporary Christians are focusing on mission for multiple purposes – both to recover tradition and to recover from tradition."
Conversations about mission and witness have become an urgent agenda for declining mainline Christians, as communions like the Anglican, United Methodist, and Presbyterian struggle to reframe their identity in a global marketplace. At the same time, adherents of new ministries often see their witness as a recovery of primitive Christianity that challenges the traditions of older denominations, she said.
Robert said the need for Christian unity is not like in the 1950s, when "self-satisfied Protestant leaders pushed for organic unity at the expense of diversity of witness."
"The growth that characterizes world Christianity today means that unity will be taken seriously only where mission is taken seriously," she said.
"Unity without witness is stagnant and oppressive. Yet witness without unity creates competitive anarchy," Robert stated.
World Christianity is at its core a theological conviction. A commitment to oneness as a mark of the church implies concrete concern for the one world and an understanding that the fate of all of God's people is intertwined, she concluded.
The huge changes in world Christianity, said another speaker, Sang-Bok David Kim, chairman of the Asia Evangelical Alliance, means that "Christianity is no longer a 'white man's religion.' Christians are now everywhere."
Looking at statistics, Kim said Christianity is still the world's largest religion, with 32.39 percent of the global population, followed by Islam at 22.90 percent.
He acknowledged, however, that Islam is growing faster than Christianity, not so much from conversion, but due to a higher birth rate in the Muslim community (1.9 percent growth, compared with Christians, at 1.2 percent).
Kim, who is also senior pastor of Hallelujah Christian Church in South Korea, said although the Global North (countries with economically developed societies) has declined in numbers overall, Evangelical, Pentecostal and Charismatic communities continue to grow there, as well as in Africa, Latin America and Asia.
The Korean pastor noted the post-World War II evangelical mission was an "astonishing success story," as was the subsequent growth that came from a new generation of indigenous evangelical movements around the world.
"Evangelicals numbered 82 million (2.9 percent) in 1960 and they have reached 546 million in 2010 (7.9 percent)," he said.
"Re-evangelization" is the main task of many churches, such as the Russian Orthodox Church, which was "concentrating more on evangelization of the 80 percent nominal Orthodox Christians," than proselytism in the 1990s, Kim concluded.
The GCF brought together leaders from all major church traditions, all theological perspectives and major world communions including Anglicans, Baptists, Lutherans, Pentecostals and other Evangelicals. More than 300 delegates from 81 countries participated in the conference.
Gabrielle Devenish contributed writing to this report.