Pope Benedict XVI has warned of an "aggressive anti-clericalism" in Spain which he said was akin to that experienced during the 1930s.
He arrived in the pilgrimage city of Santiago de Compostela at the beginning of a two-day visit to Spain.
"The clash between faith and modernity is happening again, and it is very strong today," he told reporters on the plane, quoted by AFP news agency.
He is due to celebrate an open-air Mass and then travel to Barcelona on Sunday.
"Spain saw in the 1930s the birth of a strong and aggressive anti-clericalism," Pope Benedict said.
He urged a "meeting between faith and secularism and not a confrontation" in Spain and the rest of Europe.
In a speech in Spanish in Santiago de Compostela, he called for "a Spain and Europe concerned not only with people's material needs but also with their moral and social, spiritual and religious needs, since all these are genuine requirements of our common humanity".
Correspondents say many Spaniards turned away from the church following the rule of General Franco, as democracy and secularism became synonymous.
Despite protests from the Church, the current Socialist government has ended obligatory religious education in state schools and legalised abortion on demand in a drive to secularise.
Pope Benedict was greeted at the airport in Santiago by Crown Prince Felipe and Church officials.
In Barcelona, he is due to consecrate the Sagrada Familia church, still unfinished more than a century after Antoni Gaudi designed it.
This is Benedict's second visit during his papacy, and a third visit is planned next year for World Youth Day, a sign of how important the Vatican considers the health of the church in Spain.
Only 14.4% of Spaniards regularly attend mass, and legal changes to allow divorce, gay marriage and abortion have caused concern to the Catholic Church.
But 73% of Spaniards still define themselves as Catholic.
In Santiago, the Pope prayed at the tomb of St James, the focus of pilgrimage to the city for many centuries. He will celebrate Mass outside the 12th century cathedral.
But some shopkeepers are disappointed by the number of people coming to the city for the Papal visit.
"There are still no people, we're very surprised," one souvenir shop owner told AFP news agency.
In Barcelona, gay activists are calling for a kiss-in outside the Sagrada Familia when the Pope arrives to consecrate it.
After his visit, the church will celebrate daily Masses for the first time since its construction was begun in 1882, although it is still not expected to be completed before 2026.
BBC News website readers have been sharing their thoughts about the Pope's visit:
I have just returned from seeing the Pope. People are asking very pertinent questions about the Church and what it represents. Do we really need an individual head of state, an absolute dictator in real terms, pontificating over the personal and private lives of consenting adults? I think not. I felt it my duty to protest against such a visit because I can, while others will be unable to. Errol Sari in Barcelona, Spain
The Pope's visit to Spain is very encouraging to Spanish Catholics and myself (a British Catholic who is in Spain regularly). The Church has been the source of hope and unity for the Spanish people for centuries, and will continue to be so. John Browne in Valladolid, Spain
The Church's message is really out of touch with the current times. An archaic institution threatened by the loss of power and the public funding they have had during democracy. The pope is wasting £3m of taxpayers money. Vicente in Valencia, Spain
I have been very impressed by the attitude of Pope Benedict in view of the adversity in both England and Spain. I consider his remarks on the secularism that exists in the Church today most appropriate. I was totally amazed to see what had been accomplished in the three weeks since my wife and I were in the Sagrada Familia in October. It has become a truly magnificent Basilica and an enduring credit to Antonio Gaudi. Just what is wrong with the world today that we may not glory in such a temple that honours the Holy Family and the Lord that created us? Patrick Whitaker in Alicante, Spain