How were atheists treated in Saint Petersburg (Russia) during the Great Purge ?

Being atheists was possibly a help as the Soviet regime was secular, but of more concern was whether you were considered a threat to the regime, whether someone who disliked you had informed on you to the NKVD (forerunner of the KGB) or whether you had accidentally said something derogatory about Stalin or the Soviet Union. During the purges even the suspicion that you might be a threat was enough, and even being known to be a friend of a suspect could land you in the Gulags.