If there are unbelievers in Russia that for some reason the evangelistic efforts of the Russian Orthodox church hasn't reached should we just turn a blind eye and say it's their problem? There are no political boundaries with Christ and the lines of Russia and the rest of the world have no bearing on if the gospel should or should not be preached. There are almost 200 people groups in Russia and not all of them look to the Orthodox church as their heart expression of faith.
Faith is not inherited like this, if one was born in Rome can they only be Catholic? If one was born in England can they only be Anglican? What about Constantinople... can they only be Eastern Or... ah... er... Muslim. When faith does not reach people we should not be apathetic to it and say let whatever will happen, happen.
I care less about offending other Christians than I do about bringing people to faith. Ideally, both should happen, but we just can't forget about the latter because we are too afraid of the former.