It was something else.

Your question is mostly about the current state of things, and I'm not sure that there are some historical research about it but you don't have an answer yet and I'll try to do my best.

It's hard to answer your question because it's hard to have a good definition of what is 'strong', what is 'faith', what is 'revival' and so on. I'll try to do my best, but please, bear in mind, that my definitions could be very different from yours.

First of all, atheistic approach in the USSR was relatively successful. Despite some underground movements (i.e. catacomb churches) at the end of 1970s soviet european population was mostly agnostic/atheistic, especially in the big cities. It was relatively hard and inconvenient to be a true believer because one could easily have some problems at the workplace for any church-related activities. You won't go to jail if you decided to baptise your kid, but it'd be strange to explain your party comrades why would you even think about it. So if you want to have a good career, you won't do that or you would try to do that very, very quietly.

As a result you have several generations of people who don't really know what the church is (and, mostly, don't really care). But I have to say that Russian Orthodox Church was still strong at the time because you can't kill such an organization in one day, and it had millions of true believers (who were attending churches regularly and who were reading the Bible). At the same time a lot of non-attenders would say (if you asked them) that they believe in God, and they had baptised their kids, and they're russian orthodox, but I don't know if these people should be counted at all.

But the numbers weren't so bad for the Church, anyway. In 1937 42% of the population said they belongs to the Orthodox church (link, and 1937 was a very bad year to believe in God in Russia, so the real numbers would be higher if people weren't afraid).

It seems that WWII was a factor for the Church revival. For example, there were 596 newborns in Samara (then Kuybishev) in January 1945. 356 kids were baptised the same month (not including crypto- or hidden baptismes). Hundreds of thousands were attending Easter services in Moscow (1944) and so on.

After the war (actually, during the war) life became much easier for the Russian Orthodox Church. It wasn't easy in any way, and it depended on the current ruler (Khrustschov was bad, Brezhnev wasn't so bad) but it was easier and it was safer and you wouldn't go to jail just because you read the Bible.

It all changes at the end of Perestroyka (1988-1990). I believe that it was partly based on the desire to go back to russian cultural roots (Church, Russian empire and so on). But it wasn't the only factor. The thing is, people were hungry for some spiritual guidance, and they wanted some religion in their lives. Or even magic. For example, it was the time when TV-preachers and TV-healers became very popular: you could sit before your TV set and Alexander Kashpirovsky would heal all your diseases in 15 minutes by speaking some words (video), or you could put a can with water before your TV set and Allan Chumak will charge your water with positive energy (and this water would heal all your diseases if you'd drink it — actually, it's fascinating). Books by Blavatskaya (theosophy stuff) or Rerich (Agni Yoga) or Karma's Diagnostics by Sergey Lazarev were extremely popular as well.

It was absolutely crazy but people wanted that and people were ready for that (contrary to the popular belief so-called samizdat wasn't only about dissident literature — actually, I think it was mostly about completely other stuff, and some of the most popular underground xeroxed books in the late USSR were books by Erich von Deniken and Life after Life by Dr. Moudy, so people were ready for the magic even if this magic looked completely ridiculous).

Would you call all these people orthodox or even christian? A lot of them would say they believe in God, if you'd asked them. I don't have an answer.

It's too long, so I'll try to make it shorter:

1. USSR was an atheist state, but the result of all this atheist propaganda was not an atheism per se but de-christianization.
2. A lot of soviet (and, later, russian) citizens would say they're orthodox but they're talking mostly about some primitive cultural codes — you have to baptise your kid, you have to paint eggs on Easter and eat some rice on Christmas. Oh, and also don't kill anyone, it's bad. And don't steal. And don't forget to put an icon in your car — Saint Nicholas will help you if something happens (this tradition is extremely popular even now).
3. Russian Orthodox Church is really powerful now, because it is actively supported by the Russian authorities (it wasn't a thing with the USSR, it was vice versa) and because it's the real owner of these culture codes.
4. A lot of people would say they're orthodox, but most of them haven't read the Bible so it's up to you whether to count them or not.

As the last evidence, I want to show you the results of the sociological survey from 2017 (link).

42% of people who called themselves orthodox christians don't believe in God (or not so sure about it).