"Nation" without being tied to the ethnic side is exactly the concept that we see now in some kind of France. In Africa, for example, there live Orthodox Negroes in a certain amount. Do they make it more "Russian" than the Russian atheist living in Russia?
Obviously not. The same applies to Orthodox Romanians, and Orthodox Greeks, and so on. Even the Orthodox Slavs, beloved by the whole of Russia, are not Russians by the Russians.
On the other hand, the biological side alone is not enough - we all have in mind examples of the ideological phenomena of Soviet and Ukrainian. Should still be self-identification as a Russian, loyalty to this national community. Nevertheless, Russian culture is multifaceted, it is a living and developing phenomenon, it is much more extensive than Orthodoxy alone, and it is impossible to make religious views any more determining criterion of belonging to the Russian people.
However, there is an important point worth noting. Yes, ethnicity can not be changed. The blood factor is the objective side of national identity. But that's what makes it so important. Cultural choice - amenable to change. A Communist, a Ukrainian-born Russian, may have Russian children. But having lost the national face from the biological point of view, it will not be returned by any efforts. Therefore, the preservation of the objective, biological side of nationality can not be called "unimportant" or "secondary." Because without origin, without a direct and objective blood succession with our ancestors, we are no longer a people, but simply a group of cosplayers, who with the same success can declare themselves ancient Egyptians or Annuns.