Pope Benedict XVI called Saturday for a common front with Orthodox Christians to defend traditional church values, warning of threats posed by abortion and gay marriage.
Facing discontent within his German flock, the pope said religion must not be banished from public life and that Christian churches "are walking side by side" in the battle.
"They speak up jointly for the protection of human life from conception to natural death," he told Orthodox Christians on the third day of a visit to his native Germany.
"Knowing, too, the value of family and marriage, we as Christians attach great importance to defending the integrity and the uniqueness of marriage between one man and one woman from any kind of misinterpretation," he said.
"Here the common engagement of Christians, including many Orthodox and Oriental Orthodox Christians, makes a valuable contribution to building up a society equipped for the future."
The Vatican, meanwhile, dismissed an incident earlier in the day in the city of Erfurt on the edge of the papal security zone where a man fired an air gun at a security guard about an hour before a papal Mass.
Police said the alleged shooter, a 30-year-old Erfurt man, was arrested. They said an air rifle and air pistol were found in his apartment and that he was being held on suspicion of attempting to cause serious bodily harm.
In the pope's homily in Erfurt, before a crowd of 30,000 people, he acknowledged that the collapse of communism in the former East Germany more than 20 years ago has allowed the church to function freely, but he questioned whether that change has brought any increase in faith.
"Are not the deep roots of faith and Christian life something very different from social freedom?" the pope said. "It was actually amid the hardships of pressure from without that many committed Catholics remained faithful to Christ and to the church."
After the Mass, the pope flew to Freiburg, a city with a large Catholic population and the final stop of his visit.