Religion never truly left Russia, and when his people's motivation was waning during WWII, Stalin would often use religion as a means to an end....as with all political leaders, something is only wrong until you can make some use of it.
There are various religions throughout Russia, but the main or official religion is
Russian Orthodox. One of the main differences between the Russian Orthodox and Roman Catholic Church is the use of the Julian Calender in favour of the Gregorian calender with regards to religious holy days. Also, you may notice Russian women wearing kerchiefs, this is also related to Russian Orthodox belief (similar, though not as extreme, as the muslim burka).
Dostoevsky's writings, although obviously somewhat out of date, give some interesting insights into the relationship between Russians and their religious faith.