In an article published Tuesday in the Daily Telegraph newspaper, Warsi urged Europe "to become more confident in its Christianity."
"You cannot and should not extract [the] Christian foundations from the evolution of our nations any more than you can or should erase the spires from our landscapes," she wrote.
"My fear today is that a militant secularization is taking hold of our societies," she added, accusing some atheists of having the same intolerant instincts as authoritarian regimes.
Warsi, a prominent member of Cameron's Conservative Party, is leading a delegation of British government ministers to the Vatican, where they are due to meet Pope Benedict XVI on Wednesday.
In a speech in Rome, Warsi said that "too often there is a suspicion of faith in our continent." She said in Britain religion has been "sidelined, marginalized and downgraded" and "faith is looked down on as the hobby of 'oddities, foreigners and minorities.' "
Warsi's words echo comments by the pope, who visited Britain in 2010 and warned of the spread of "aggressive forms" of secularism.
The Vatican appeared to approve of Warsi's speech. In a break with the usual protocol, it emailed the text to correspondents in Rome.
But Evan Harris, a former Liberal Democrat lawmaker and vice president of the British Humanist Association, said Warsi's talk of militant secularism was "self-serving paranoia."
"There is nothing militant about calling for an end to blasphemy and apostasy laws or wanting religious persecution of women and gay people to end," he said. "Secular liberal democracy, which involves the separation of church and state and an end to religious privilege, is the best guarantor of religious liberty and free expression."
Secularists object to state funding for faith schools, whose numbers have increased under recent governments. There are state-funded Jewish, Muslim, Hindu and Sikh schools, as well as thousands of Church of England and Roman Catholic schools. All follow the same curriculum as nonfaith schools, but can teach their own views in religious studies classes.
While American political candidates often talk openly, even boastfully, about the role religion plays in their lives, British politicians usually avoid deep professions of faith.
Blair, prime minister between 1997 and 2007, is a committed Christian, but rarely spoke about his religion while in office, and waited until he left power to convert from Anglicanism to Roman Catholicism.
Cameron, the current prime minister, has said his experience of Christian faith is like the signal on a faulty radio: "It sort of comes and goes."
His deputy, Liberal Democrat leader Nick Clegg, is an atheist, as is Ed Miliband, leader of the main opposition Labour Party.
In the United States, Republican presidential candidates currently battling for their party's nomination routinely emphasize their religious credentials while accusing President Barack Obama of taking anti-religious stances.
Newt Gingrich recently accused Obama of "waging war on religion" while Rick Santorum said the president's policies were challenging America's traditional freedom of religion.
In Britain, God is rarely considered a vote winner though comments by Warsi and others suggest that might be changing.
Cameron recently urged the Church of England to lead a revival of traditional Christian values to counter the "slow-motion moral collapse" that led to the August riots in England.
This week Communities Secretary Eric Pickles condemned a High Court ruling that a town council in southwest England must stop holding prayers at the start of meetings.
"We are a Christian country," Pickles said.
Traditionally, that is true. The Church of England is the country's established church, with Queen Elizabeth II as its temporal head. Bishops help make laws as members of the House of Lords.
In the 2001 census the last for which full results are available just under 72 percent of people identified themselves as Christian. But most Britons are not regular churchgoers, and many see Christianity as a loose cultural identity rather than a strong faith.
Steven Fielding, a professor of politics at the University of Nottingham, said praising religion fitted with the moderate Cameron's vision of a "Big Society" in which charities and community groups will take over some functions of the state.
"It's also a useful issue," Fielding said, "to indicate to his backbenchers and those out there in Daily Telegraph-land" readers of the conservative newspaper "who question whether he is a true conservative."
But Fielding said religion was unlikely to reach U.S. levels of political importance in Britain.
"People like to think their leaders have got, in the same way they have, a vague belief in God, that they go to church occasionally," Fielding said. "But it's potentially a divisive and dangerous issue if it's taken to anything other than talking in generalities."
Associated Press writers Gregory Katz in London and Nicole Winfield in Rome contributed to this report.