The Pope has warned that religion - and Christianity in particular - is "being marginalised" around the world.
His comments came in his keynote speech to UK MPs, senior members of British society, and religious leaders at Westminster Hall in central London.
Pope Benedict XVI warned that there were some people who wanted to see "the voice of religion be silenced".
He returned to the subject in a service at Westminster Abbey, asking Christians to speak out about their faith.
BBC correspondent Peter Hunt described the speech at Westminster Hall as "a rallying call, and a plea - for religion not to be squeezed out by secular society".
The Pope was speaking as a sixth man was arrested as counter-terrorism detectives investigate an alleged threat to Pope Benedict XVI's visit.
In his speech at Westminster Hall, the Pope called on those in attendance to seek ways to promote faith "at every level of national life".
He added: "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."
At the service celebrated jointly with the Archbishop of Canterbury, Dr Rowan Williams, at Westminster Abbey, the Pope said: "in a society which has become increasingly indifferent or even hostile to the Christian message", believers were "all the more compelled to give a joyful and convincing account" of their faith.
He also shook incense over the tomb of Edward the Confessor - regarded as a saint by both Churches.
Following the service, the Pope travelled back to the home of Papal Nuncio - his official representitive in the UK - in Wimbledon, south-west London.
Earlier, the Pope met Dr Williams at Lambeth Palace - a meeting attended by Anglican and Roman Catholic bishops from different parts of the UK.
It comes at a time when Anglican moves towards appointing women bishops have unsettled relations with Rome.
But the Pope said he wanted to focus on the "deep friendship" between the two churches rather than their differences.
The Lambeth Palace meeting marked the first time a Pope has met the Archbishop at his official residence.
It was viewed as an important event more than 40 years after official talks began about possible reunification of the two churches.
Divisions remain over Roman Catholic opposition to the ordination of women priests. Meanwhile the Church of England's General Synod left the way open for appointing women bishops at its meeting in July.
The Vatican angered many supporters of women's ordination by describing it as a "grave crime" to be dealt with in the same process as sex abuse, though it denied it was equating the two.
Dr Williams said in his opening remarks that the Pope was "most welcome" at Lambeth Palace.
He then praised the pontiff's "consistent and penetrating analysis of the state of European society".
As the Pope left Lambeth Palace to travel the short distance to Westminster Hall in his Popemobile, thousands of people lined the streets.
At one point he stopped the vehicle to bless a baby that was handed up to him.
Earlier, the Pope led an assembly of 4,000 Catholic school children in west London.
He used his address at St Mary's University College in Twickenham, to say that the safety of children was vital in all schools, an apparent reference to the Church's child abuse scandal.
He also warned against the limitations of celebrity and science.
The pontiff told the crowd young people were often encouraged to model themselves on celebrities.
He said: "My question for you is this: What are the qualities you see in others that you would most like to have yourselves? What kind of person would you really like to be?'
"I'm asking you not to be content with second-best."
He went on to say having money or a successful career was not enough to make people happy, but true happiness could be found in God.
Tight security surrounded the event, called The Big Assembly, and monks and nuns who had waited in long queues were frisked by police.
Around 100 protesters against the Vatican's record on gay rights, equality and birth control had gathered ahead of his arrival, amid tight security.
The Church saw it as an opportunity to celebrate the work of more than 2,000 Catholic schools across the UK, in partnership with the state.
But critics said it could fuel hostility to faith schools and serve as a painful reminder of the child abuse scandal within the Catholic Church.