Currently, there is no official census of religion in Russia, and estimates are based on surveys only. In 2012 the research organization Sreda published Arena Atlas, a detailed enumeration of religious populations and nationalities in Russia, based on a large-sample country-wide survey. They found that 46.8% of Russians declared themselves Christians (including 41% Russian Orthodox, 1.5% simply Orthodox or members of non-Russian Orthodox churches, 4.1% unaffiliated Christians, and less than 1% for both Catholic and Protestant), while 25% were spiritual but not religious, 13% were atheists, 6.5% were Muslims, 1.2% were followers of "traditional religions honoring gods and ancestors" (including Rodnovery, Tengrism and other ethnic religions), and 0.5% were Tibetan Buddhism. However, later that year, the Levada Center estimated that 76% of Russians were Christians, and in June 2013, the Public Opinion Foundation estimated that 65% of the population was Christian. These findings are in line with Pew Research Centers 2011 estimate that 73.6% of Russians were Christians, with Russian Public Opinion Research Center (VCIOM)s 2010 survey (~77% Christian), and with Ipsos MORIs 2011 survey (69%).
Question: Which group did a study that had the highest percentage of Russians listed as Christians?
Answer: Russian Public Opinion Research Center