Doug Birdsall told thousands of evangelicals at the opening ceremony of Cape Town 2010 that conversation on a global scale was required if Christians were to find solutions to the problems facing the world and the church today.
He said there was an urgent need for the church to "be authentic" and discern effective ways of presenting the Gospel to the people of the world, particularly those of different faiths.
"I pray that this conference – described as the most diverse gathering of Christians ever – will be reported on as the most united expression of the church that the world has ever seen," he said.
"Authentic community is a precondition of authentic prophetic witness in our world."
Remaining obstacles in efforts to evangelise the world included, he said, the work of translating the Bible into all spoken languages and penetrating the last of the world's unreached people groups.
He recalled the milestone made by the late Ralph Winter at the first Lausanne Congress in 1974, when the missiologist introduced the term 'unreached people groups' to the world church.
"How can it be that 2000 years after the time of Christ and 36 years after the call to take the Gospel to all nations at that Congress there are still those who have not heard?" he said.
"We pray that by the time there is a next Lausanne Congress that the number of unreached people groups will be at zero.
"We pray that the church will also be mobilised so that those at the forefront of Bible translation say that the number of people groups with no Scripture in their own language is zero."
Birdsall expressed his regret over the absence of several leaders of the house church movement in China who were prevented from attending by the Chinese authorities.
"God make them a blessing in their hour of need and their hour of a sense of isolation … when their heart's desire was to be here with us," he said.
He remained upbeat however about the prospects for world evangelisation and encouraged delegates to listen to God's voice during the Congress.
"We have come to dream. We believe that the future is as bright as the promises of God."
The opening ceremony took on a distinctly African flavour, with the music and singing regionally inspired and drama performances highlighting some of the developmental challenges still facing the continent.
Birdsall said that the conference would be "global in scope but African in flavour and nuance".
"We want the global church to experience the vitality of the church on this continent - the aspirations, the challenges, the sorrows – and so to be blessed by it. And in turn we want the global church to be a source of blessing," he said.
Archbishop Henry Orombi, head of the Church of Uganda, welcomed delegates to the continent. He said the fact that the Lausanne Congress was taking place on African soil was a "miracle".
Around 4,000 Christians are meeting in Cape Town to discuss evangelisation in the 21st century.
Thousands more are taking part in smaller congresses being held around the world at "Global Link" sites, and as many as 200,000 are expected to take part online.
As well as being the first to go viral, Cape Town 2010 is the first Lausanne Congress to be held in collaboration with the World Evangelical Alliance.
WEA Secretary General Dr Geoff Tunnicliffe said he hoped Cape Town 2010 would become a "significant catalyst" for the advancement of God's kingdom.
He told delegates of the need for humility and partnership among Christians in the face of unprecedented global challenges, including the spread of radical secularism and religious extremism, the HIV and AIDS pandemic, and environmental degradation.
He said: "We must come together to face these challenges. Our commitment in seeking to fulfil God's purposes demands greater levels of partnership between all of us."
Messages from the founding fathers of The Lausanne Movement, Billy Graham and John Stott, were read out during the opening ceremony.
Stott spoke of his joy in the growth of the church in the majority world since Lausanne '74 and urged delegates to "live a life worthy of the calling you have received".
Graham urged delegates to assess the changes that had occurred in the world since the first Lausanne Congress and "rededicate" themselves to the urgent task of evangelism.