Some priests too.
Part of why ROCOR formed was because they felt that the Russian Orthodox Church had lost Grace when Patriarch Sergius aligned with the government. I try not to judge him too harshly because he was in a tough position. Similar ROCOR-esque organizations formed as other countries fell to Communism (for example, the Free Serbian Orthodox Church) and after Communism fell, most of these organizations reunited with their Mother Church.
Now, you also had what is called the "underground Church" in the USSR. While the government did allow a number of parishes to operate, most of the ones that did were filled with untrustworthy priests who were indeed spies. The "underground Church" were those who worshiped in secret.