Professor Dana Robert, of Boston University School of Theology, told some 300 Christian leaders at the Edinburgh 2010 conference that the greater plurality of the world church today made united witness "urgent for the integrity of the gospel message".
"We must not allow difficult theological, socio-cultural, and political issues, or disagreements over theologies of religion, to discourage us from sharing God's love and salvation through Jesus Christ with all the world," she said.
Mission work today, she suggested, needed to be grounded in the Revelation vision of people from all tribes and nations witnessing to Christ in unity, while the concept of "the whole church taking the whole gospel to the whole world" implied a need for stewardship of God's Earth.
"As a native of southern Louisiana and the grandchild of Gulf Coast fishermen, I am grieving the destruction of my homeland from Hurricanes Katrina and Rita, and now the massive oil spill inundating the marshes.
"We must work for the 'repair' or the 'salvation' of God's creation when we conceptualise mission in the 21st century."
The Edinburgh 2010 conference is taking place this week to celebrate 100 years since the historic World Missionary Conference took place in the Scottish capital in 1910. That gathering brought together around 1,200 Protestant missionary leaders mostly from North America and northern Europe around the theme of evangelising the whole world "in this generation".
Professor Robert paid tribute to delegates at the 1910 conference like Bishop V S Azariah, Beijing church pastor Cheng Jingyi and chairman John R Mott, who championed the view that Christian mission could not be separated from the work of Christians to build unity among believers of different nations, ethnicities, denominations and social classes.
It was Azariah who most famously pleaded "Give us friends", a reference to the distance commonly felt between foreign missionaries and indigenous believers.
"The courage behind Azariah, Cheng and Mott's statements lay in their eschatological hope," she said.
"Obviously in human terms they could not achieve the Revelation vision. Colonialism and racism continued unabated. World War I broke out soon afterward.
"But they lived in courageous certainty of the 'long view' that God's love would ultimately prevail."
Professor Robert invited Christians to see themselves as part of the "larger narrative" of God's ongoing work on Earth and be encouraged by the knowledge that being a witness of the Good News meant not only witnessing to what God had done in the past, but also to what he was doing in the present and would do in the future.
"The biblical promises draw us forward and make united witness to Christ both possible and necessary," she said.
"The multiplicity of 2010 celebrations around the world should be affirmed as signs of hope and opportunities new forms of mission that must engage each other.
"The essence of worldwide Christian community is being reimagined as a more inclusive and broader 'global conversation' than was possible in the past."
Pointing to the English, West African and German missionaries who stood against the slave trade two hundred years ago, she said Christians today needed to "proceed in assurance that despite all odds, our united witness across multiple human boundaries makes a difference in the world".
She concluded: "As witnesses to Jesus Christ today and led by the Holy Spirit, to take the 'long view' means that we act on the promises that there will be no more hunger or thirst, that springs of living water will flow, and that God will wipe away the tears in the eyes of the world he loves.
"Even as we ask 'How long, O Lord, how long?', united in praise, we confidently embrace God's mission."