Pope Benedict XVI said the UK is a "force for good" today as he arrived for a historic visit.
But he also delivered a warning about "aggressive forms of secularism" when he urged the nation not to lose its traditional values as it "strives to be a modern and multicultural society".
The pontiff was officially welcomed by the Queen at the Palace of Holyroodhouse in Edinburgh for the first papal state visit to the UK.
In a speech at the palace, delivered in English, the Pope spoke of the UK's important place in history.
He said: "Your forefathers' respect for truth and justice, for mercy and charity, come to you from a faith that remains a mighty force for good in your kingdom, to the great benefit of Christians and non-Christians alike."
He cited anti-slave campaigners William Wilberforce and David Livingstone, and women such as Florence Nightingale, as examples of that force for good.
And he praised Britain's fight against Hitler's "atheist extremism", saying that "Britain and her leaders stood against a Nazi tyranny that wished to eradicate God from society and denied our common humanity to many, especially the Jews, who were thought unfit to live".
The Pope, who was forced to join the Hitler Youth as a 14-year-old schoolboy, said the UK remained "a key figure politically and economically on the international stage".
"Your Government and people are the shapers of ideas that still have an impact far beyond the British Isles. This places upon them a particular duty to act wisely for the common good."
And, referring to the future, he delivered an apparent warning about the risks to the nation's traditional values.
He said: "Today, the United Kingdom strives to be a modern and multicultural society.
"In this challenging enterprise, may it always maintain its respect for those traditional values and cultural expressions that more aggressive forms of secularism no longer value or even tolerate."
The highlight of the visit for Catholics will be the Pope's beatification of Cardinal John Henry Newman - the 19th century cleric who converted from the Church of England.
But the trip comes amid renewed anger at the worldwide child abuse scandal that has engulfed the Roman Catholic Church and dogged the Pope's own religious career.
That anger has gathered pace with recent revelations in Belgium of hundreds of new victims, at least 13 of whom committed suicide over the years.
As he flew to Scotland, the Pope spoke of his "sadness" over his church's handling of child abuse scandals.
He said the Catholic Church had not dealt with abusive priests decisively or quickly enough and said its top priority now was helping victims to heal and to regain their trust in the church.
The Pope's comments, to reporters on board his plane, marked his most thorough admission to date of failings in the way the sex abuse scandal was handled.
He also spoke of protests planned for his UK visit, saying Britain had a "great history of anti-Catholicism. But it is also a country with a great history of tolerance".
Asked about polls that suggest the faithful had lost trust in the church as a result of the sex scandals, he said he was shocked and saddened by the scope of the abuse, in part because priests take vows to be Christ's voice upon ordination.
He said he felt "sadness also that the church authority was not sufficiently vigilant and not sufficiently quick and decisive to take the necessary measures" to stop the abuse and prevent it occurring again.
Victims abused by priests, and secular campaigners, have called on the pontiff to go further than an apology and hand over all information about suspected abusers within the church.
The Pope is widely expected to meet some of those who suffered during his visit to Britain.
It is the first papal trip to Britain since Pope John Paul II made a pastoral visit in 1982 following an invitation from the Church.
Thousand lined the streets as the Pope was driven through Edinburgh, and past the controversial Scottish Parliament building, to the Palace of Holyroodhouse for his meeting with the Queen.
Sister Francis, who travelled to the city with members of the Carmelite Monastery in Fife, was one of thousands on the streets.
She said the visit will help bring faiths together and added: "I think it will be a blessing, whether people know it or not. It will improve relations between Church and state and between other religions."