In the Soviet Union, the Lenin / Stalin types promised food, shelter and a better life to all if they adopted communism. Once they took power, the first thing they did was a series of purges. Anyone who joined the communist party out of opportunism was killed or sent to a labor camp. Anyone who joined but didn't adopt 100% of the party platform was killed or sent to a labor camp. They had taken control and the next step was purity control.
Had you lived in those times, you would have supported the revolution and then once it was successful, you'd have probably been taken back out behind the garage and shot in the head.
When you experience people on the left attacking you for not being 100% true to party, that is where it is coming from.
https://en.wikipedia.org/wiki/Great_Purge