Pope Benedict XVI appealed to believers not to leave the Catholic Church as he arrived for a four-day visit to his native Germany.
The Pope said on the flight from Rome that he understood why some people – especially sex abuse victims and their loved ones – might say "this is no longer my Church".
But he urged Catholics to see the Church was made of both good and bad, and was struggling to right the wrongs committed in its ranks.
Germany has been rocked by the clerical sex scandals that have swept across Europe in the past two years.
"The Church is a net of the Lord that pulls in good fish and bad fish," he said. "We have to learn to live with the scandals and work against the scandals from inside the great net of the Church."
A record 181,000 German Catholics officially quit the Church last year.
The Church in Germany has received almost 600 requests for compensation for victims of sexual and physical abuse, while a victims' association estimates that more than 2,000 people were mistreated by Catholic priests in recent decades.
The Pope was met at the airport by Christian Wulff, the German president – a Catholic who has divorced and remarried in defiance of Church rules – and Chancellor Angela Merkel.
In an interview published on Thursday, Mr Wulff said he hoped Benedict would loosen some of those rules.
"Millions of people in mixed Catholic-Protestant marriages and millions of divorced and remarried Catholics, as well as other groups, expect satisfactory messages," he told the Catholic news agency KNA.
He also said he hoped the pope would say positive things about relations with Jews and Muslims.
Later in the day, the pope was due to hold talks with Merkel, address the Bundestag lower house of parliament and celebrate an open-air Mass before about 70,000 Catholics at Berlin's Olympia Stadium.
The Pope's two previous visits home since his 2005 election were to the mostly Catholic regions of the Rhineland and his native Bavaria. This trip takes him to the mostly Protestant and atheist eastern part of Germany, where he can expect more criticism of his conservative policies.
Thousands of protesters are expected to voice their opposition to his visit and about 100 deputies have said they will boycott his address to parliament because they say it violates the separation of church and state.
On Friday, Benedict will travel to the eastern city of Erfurt to meet Protestant leaders in the monastery that once housed the 16th century reformer Martin Luther, whose teachings led to Western Europe's split between Catholics and Protestants.
Germany's Protestants, about equal in number to Catholics, hope Benedict will signal an opening towards some of their wishes.
Protestants and some Catholics have urged him to allow joint communion services and let Protestant spouses of Catholics receive the Eucharist at Catholic Mass.
They will also listen carefully to what he says about Luther, who split western Christianity in 1517 with his 95 Theses against Catholic practices of the day, such as the Church accepting payment to absolve sin.
The Protestants want to celebrate the 500th anniversary of that event in 2017 with nationwide celebrations emphasising Luther's contributions to Christianity and German culture.
How Benedict speaks of Luther should give an indication of how the Catholic Church plans to mark that date.
After talks with Protestant leaders in Erfurt, he will travel to nearby Etzelsbach to meet a small Catholic community that withstood persecution under East Germany's officially atheist communist rule.
He will end his German tour in the mostly Catholic southwestern city of Freiburg.