First, this is not the case, the goal of Marxism is to control as many slaves as possible. There never was interest in population control. See below:
Tenets of Marxism:
Straight from the Communist Manifesto:
1. Expropriation of landed property, and the use of land rents to defray state expenditure
2. A vigorously graded income tax
3. Abolition of the right of inheritance
4. Confiscation of the property of all emigres and rebels
5. Centralization of credit in the hands of the State, by means of a national bank with state capital and an exclusive monopoly
6. Centralization of the means of transport in the hands of the State
7. Increase of national factories and means of production, cultivation of uncultivated land, and improvement of cultivated land in accordance with a general plan
8. Universal and equal obligation to work; organization of industrial armies, especially for agriculture
9. Agriculture and urban industry to work hand-in-hand, in such a way as, by degrees, to obliterate the distinction between town and country
10. Public and free education of all children. Abolition of factory work for children in its present form. Education and material production to be combined.
Nowhere is population control or reduction mentioned.
Misunderstanding this leads you to conclude that Russia is still Marxist or Communist. There is much evidence that it is not, wants the population of ethnic Russians and Slavs to grow, and does cultivate Christianity.
That cultivation does not mean that devout Christianity will rapidly reemerge, only that it can.
Your concern that only a small percentage of Orthodox Christians are devout is a valid concern. Considering what Russians are recovering from we will have to be patient to see how Christianity recovers there.
One should not put forth the communist manifesto to claim that Marxists today do not have population reduction as a no. 1 priority.
The Population Control Agenda
http://www.radioliberty.com/pca.htm
Russia is moving toward full-blown atheistic communism again...not that they ever fully left it. The people have forgotten just how bad it was...so very quickly.
http://www.forbes.com/sites/kenrapoza/2011/12/06/can-russias-communist-party-make-a-comeback/