A day after arriving in Bavaria, the pope led a mass for up to 250,000 people in Munich, the first of three he will take during his six-day pilgrimage to his southern German homeland.
He told the predominantly German crowd that there was a growing danger of a "hardness of hearing where God is concerned".
"This is something from which we particularly suffer in our own time. Put simply, we are no longer able to hear God -- there are too many different frequencies filling our ears," he said.
The 79-year-old pontiff has said the visit to Bavaria is a trip down memory lane but it has the more serious purpose of attempting to arrest the decline of religious observance in Germany.
More than 100,000 people are said to leave the German Catholic Church every year.
Pope Benedict said Western societies are losing their souls to scientific rationality and frightening believers in the developing world who still fear God.
He said that spreading the word of Jesus Christ is more important than all the emergency and development aid that rich churches like those in Germany give to poor countries.
He criticised the Catholic Church in Germany for its reluctance to evangelise, saying that while it was always ready to help dioceses in Africa or eastern Europe to rebuild churches or embark on social projects, it seemed reluctant to seek converts to the Catholic faith.
He also stressed the role of faith in fighting AIDS "by realistically facing its deeper causes", indirectly confirming the Church view that premarital abstinence and fidelity in marriage are the way to combat sexually transmitted diseases.
Mass well received
His message went down well with the Bavarian faithful.
Renate Boml, 45, said she got up at dawn to catch a train from her village to attend the mass.
"The pope has pleasantly surprised me with his personal warmth and his intelligence," she said.
"We are happy to be here for this celebration of our beliefs."
The pontiff was driven to the mass in a pope-mobile under clear blue skies.
Later on Sunday he celebrated vespers in Munich Cathedral, a familiar location for the former Cardinal Joseph Ratzinger who was archbishop of the city from 1977 to 1982.
He said spirituality and Catholicism should have a higher priority in education, and called on parents to "help your children to believe".
He will also hold mass in the town of Alltoeting on Monday and in the city of Regensburg on Tuesday.
This visit bears the personal stamp of the pope, who said on his arrival that his heart "beats with a Bavarian rhythm".
House daubed
On Monday, he will visit the house in the small town of Marktl-am-Inn where he was born on April 16, 1927.
The yellow and white-painted exterior of the house was being hastily cleaned after blue paint was thrown at it early on Sunday.
Police said they believed it was the work of vandals and not a protest.
The pope will spend Wednesday with his brother, Monsignor Georg Ratzinger, in Regensburg and together they will visit the graves of their mother, father and sister.
Pope Benedict is making his fourth foreign trip since becoming pope in April last year and the second to Germany in that time.
He is due to visit Turkey in November and Brazil next year.
Security for the German trip is tight in the wake of the attempted bombings of two passenger trains in two western cities in July. Around 5,000 police are on duty.