The Orthodox Christian religion is being made a mandatory school subject in four of Russia's regions, as supporters claim that it will help protect traditional spiritual values in the country.
Pupils in the Belgorod, Bryansk, Kaluga and Smolensk regions will be taught the basics of Orthodox Christianity.
It will also be included as an optional subject in the school curriculum in 11 other regions across the country.
However, critics say it violates the constitution of the secular state.
Although Orthodox Christianity is Russia's main religion, the country's Muslim community makes up more than 10 percent of the total population.
Responding to the introduction of the subject, the central educational watchdog body, Rosobraznadzor, said the church was separate from the state, so the basics of Orthodox Christianity should only be taught as an optional subject.
But the introduction of the new subject comes after lawmakers in the 15 regions backed the move.
Russian Education Minister Andrei Fursenko also voiced support, saying "schoolchildren must know the history of religion and religious culture".
He said it was a matter for the regions to decide.
"This year, a textbook on the history of world religions is available for the first time. It pays a lot of attention to Russian Orthodox Christianity," he said.
Muslim leaders have responded by saying that they will ask for lessons on Islamic culture to be extended.