Russian Orthodox Patriarch Kirill hailed members of the Church this past week for remaining firm in their belief in God in the face of "aggressive atheism" and "resurgent paganism."
Kirill made the remark during celebrations to mark Russia's conversion to Christianity on Wednesday – the day declared last month by Russian President Dmitry Medvedev to be a national holiday.
The Russian Orthodox leader, who was visiting Ukraine at the time, also told reporters in Kiev's historic Pecherska Lavra monastery that "[a]bandoning the historical significance of the baptism of Rus means discarding the supporting pillar of our entire civilization."
Orthodox Christianity was introduced as the state religion in ancient Rus in 988 when Vladimir the Great was baptized and later converted his family and other people.
President Medvedev decreed the new official holiday on June 1 amid strong criticism from rights groups and activists who said it goes against Russia's secular constitution.
Muslims, who make up one-seventh of the Russian population, complained that the holiday excluded them. And Konstantin Bendas, a senior official with the Russian Union of Christians of the Evangelical Faith, said the national holiday created tensions between the Orthodox Church and other faiths, according to the St. Petersburg Times.
Despite the controversy, many were supportive of the idea.
Vsevolod Chaplin, head of the Moscow Patriarchate's department for church and society affairs, said "Russia is an Orthodox state, and we should not be ashamed of declaring it."
Prime Minister Vladimir Putin, meanwhile, said the conversion of Russia to Christianity was of great significance and a "historical choice."
Religious ceremonies and mass baptisms in rivers were held across Russia on Wednesday as well as Ukraine, where Patriarch Kirill was concluding an eight-day visit.
More than 60 percent of Russians describe themselves as Orthodox Christians, while Muslims make up more than 10 percent of the population.