Carver Yu, President of the China Graduate School of Theology in Hong Kong, said that "confusing ideologies" were creating emptiness and alienation among people, while indifference to religion was "tightening its grip".
He said the recent advertising campaign by Richard Dawkins and other atheists on London buses was a perfect example of the "enthusiastic zeal" with which atheists were campaigning against Christianity and religion.
"Atheism is about to become the new religion," he said. "Christians must preach the Gospel of Jesus Christ fearlessly because he is the way, the truth and the life. Only he can lead us away from the present state of godlessness."
Yu was addressing more than 4,000 evangelical leaders from around the world at Cape town 2010, the Third Lausanne Congress on world evangelisation taking place this week in the South African city.
Also taking to the podium was Michael Herbst, researcher in evangelism and church development. He warned that the decline of faith among parents was leading to a whole generation of children in Germany growing up with an "atheist mindset" and the belief that "faith doesn't matter".
He noted that the idea of a "singular truth" and monotheism had become unpopular and were widely regarded as dangerous, arrogant and potentially violent.
"Everything is relative now except for this one new and ultimate truth - that there is no singular truth," he said.
"All those professing a singular truth should be silent in a tolerant world."
Os Guinness, co-founder of The Trinity Forum, echoed his sentiments. He said that the biblical view of truth had become "obscene to modern minds" and was being taken by many to be exclusive, intolerant and divisive.
"But on a deeper look the biblical view is profound, timely and urgent for the day, even for those who reject it," he said.
Guinness was especially critical of liberal Protestants who he said had been "careless" with the truth. He contended that they were just as "dangerous" as those outside of the church and contributing to a weakening of the faith.
Guinness said Christians had a responsibility to confront false ideas and beliefs and uphold the truth of the Gospel.
"Our stand for truth must start in the church itself. We must resist the powerful seductions of those who downplay truth for methodology, or truth in the name of activism, or truth for entertainment, or truth for seeker sensitivity, and above all those who put a modern and revisionist view to the truth in the place of a biblical view."
He continued: "Shame on those Western Christians who casually neglect or scornfully deny what the scriptures defend and what many brothers and sisters would rather die [for] than deny – that Jesus is the way, the truth and the life."
Herbst said Christians could not stay silent about their faith or be content with personal truth only. He encouraged Christians to share their faith "from below" by serving and living out truth in their own lives.
"It is not the prevalence of Christendom that empowers the Gospel but the power of the Holy Spirit that brings the truth into the heart of those who listen," he said.
Truth is the first of six main themes to be discussed at the Congress over the next seven days. Other issues on the agenda include evangelism in a multi-faith context and the integrity of the church.
More than 4,000 Christians are in Cape Town for the Congress, with hundreds of thousands more expected to take part online and at smaller congresses being held at 650 "Global Link" sites around the world.