With Putin talking about the orthodox church being holy and using to justify some conservative bullshit, all I can remember was the bolsheviks going around the country proudly declaring that cities were now "officially godless" and redistributing the church's gold amongst the people and using it to pay for electrification projects.
This is a good explanation for the very sudden rise in the 90s, alongside people discovering religion as a guideline in the chaos, pauperization and uncertainty following the collapse. But it has gone up continuously and almost without interruption after that. If it was just "i actually was always devout, i just had to hide it under the Soviets", you'd see it plateau after a few years, but that plateau phase is only reached 23 years after the USSR's fall. Active proselytization seems more likely as an explanation for the rise past the mid 90s.
I mean, this is just guesswork based on what seems plausible at a glance, i'm not particularly knowledgeable on Russia.