The USSR was trying to implement state atheist, but the majority of the people were religious. That said, the USSR did their best to reduce the power of the church, and it did it almost 100%. Confiscated all church property, didn't build new churches, and while you were free to pratice your religion in private and in churches/mosques/synagouges , public displayes were not allowed. A lot of programs were introduced to promote atheist and science over religion (as if the two relate..). To speak of your religion was strictly not allowed. They how ever used religion sometimes for propaganda purpose to unite the people, but rarely. Over times generations did become more and more atheist, but the religious ones didn't go away. Now, how did it go back to being religious (compared to others)? Well, when the situation went to shit, and the USSR collapsed, it left a lot of people in a horrible state. This was easily used by the church, and a lot of people really did find hope in religion, even blaming godlessness of communism for the bad things that happened. But we shouldn't act like the atheist education of 70 years did nothing. Russia has a large amount of atheists still, and a surprisingly low amount of Christians compared to population. By that i mean around 50%, with 20-30% not being religious (not necceceraly atheist). Religion is steadily rising, but it is not quite "recovered" from the Soviet attempt to make it powerless