State atheism was good actually. The orthodox church didn't stop with their anticommunism before their marginalisation, during their marginalisation or after they were invited back. No amount of grace extended to the Orthodox or Catholic churches by communist movement has been received in good faith or with gratitude or had ever stopped them from trying to quell communism.

Lenin said that the church is nothing but an instrument of bourgeois reaction, and the church has spent a century proving him right.

In retrospect, they should've just kept the religious status quo but made every single member of the clergy be a card-carrying member of the CPSU subject to internal party discipline while banning all other "independent" churches. Religious dogma should incorporate things like "Jesus said communism rules and capitalism drools" and "the bourgeoisie go straight to hell when they die because the bourgeoisie are not human." Christians who don't accept Jesus's teaching that communism is based get excommunicated and send to hell where the rest of the bourgeoisie are burning as well.

Overall, China handled religion a lot better. There isn't a hard push for state atheism (although I think the CPC is officially an atheist organization), but all religious groups have to swear loyalty to socialism with Chinese characteristics and submit themselves to the will of the socialist state. Those that don't toe the socialist line like Falun Gong have the hammer dropped on them.

The way the CPC handled Tibetan Buddhism is an illustrative example. Instead of abolishing a particular branch of Buddhism that used the skins of serfs for their drums, the CPC incorporated Tibetan Buddhism within the state apparatus. Whenever some religious leader dies and a new "reincarnation" has to appear to take their place, by sheer coincidence, the "reincarnation" who eventually appears is someone who the CPC liked the most.

This is the way to go. People need to quit being Reddit atheists and think strategically about this.

While I agree it was an error, it's hard for me to say it was the biggest.

In what way?