Cardinal Murphy-O'Connor urges deeper understanding between believers and non-believers
The Archbishop of Westminster has urged Christians to treat atheists and agnostics with "deep esteem".
Believers may be partly responsible for the decline in faith by losing sense of the mystery and treating God as a "fact in the world", he said in a lecture.
Cardinal Cormac Murphy-O'Connor called for more understanding and appreciation between believers and non-believers.
But the leader of Roman Catholics in England and Wales said that Britain must not become "a God-free zone".
The cardinal's lecture at Westminster Cathedral comes after a spate of public clashes over issues such as stem-cell research, gay adoption and faith schools.
Mystery of God
He expressed concern about the increasing unpopularity of the Christian voice in public life, saying: "Our life together in Britain cannot be a God-free zone and we must not allow Britain to become a world devoid of religious faith and its powerful contribution to the common good."
Later, he told BBC Radio 4's Today programme why he thought it was dangerous to be governed by reason alone.
He said saying that "supposedly faithless societies" ruled only by reason were like those created by Hitler and Stalin, ripe for "terror and oppression".
Last year, Cardinal Murphy-O'Connor complained of a "new secularist intolerance of religion" and the state's "increasing acceptance" of anti-religious views.
To stem this tide, he said Christians must understand they have something in common with those who do not believe.
God is not a "fact in the world" as though God could be treated as "one thing among other things to be empirically investigated" and affirmed or denied on the "basis of observation", said Cardinal Murphy-O'Connor.
"If Christians really believed in the mystery of God, we would realise that proper talk about God is always difficult, always tentative.
"I want to encourage people of faith to regard those without faith with deep esteem because the hidden God is active in their lives as well as in the lives of those who believe."
But Richard Dawkins, scientist, staunch atheist and author of books including The God Delusion, told BBC Radio 4's Today programme that the cardinal's comments carried no weight.
Referring to God as an "imaginary friend", Mr Dawkins said: "When talking to a politician you would demand proof for what they say, but suddenly when talking to a clergyman you don't have to provide evidence.
"There's absolutely no reason to take seriously someone who says, 'I believe it because I believe it.'
"God either exists or he doesn't. It's a matter of the truth."
Speaking later on Radio 4, Cardinal Murphy-O'Connor answered those criticisms. He said: "I think there are a number of people in this country who would like to marginalise religion.
"They would much prefer not to see religion as neutral, but to neutralise it.
"And there are unbelievers who construct their own God in order to demolish him."
Of claims that faith has no basis in reason, he replied: "To believe in God is not unreasonable."