Pope Benedict XVI warned of the "marginalisation" of Christianity during an address to dignitaries in Westminster Hall.
Speaking as part of his visit to the UK he called on those in attendance to seek ways to promote faith "at every level of national life".
The Pope said: "I cannot but voice my concern at the increasing marginalisation of religion, particularly of Christianity, that is taking place in some quarters, even in nations which place a great emphasis on tolerance.
"There are those who would advocate that the voice of religion be silenced, or at least relegated to the purely private sphere.
"There are those who argue that the public celebration of festivals such as Christmas should be discouraged, in the questionable belief that it might somehow offend those of other religions or none."
In attendance were the Deputy Prime Minister Nick Clegg, and former prime ministers Gordon Brown, Tony Blair, Sir John Major and Baroness Thatcher.