Unfortunately, the Soviest government itself was responsible for the stuff that went on, though admittedly there was a lot more playing into it than religion, especially the Soviet Leader's (Stalin, Lenin, etc.) fears that there would be a revolution which would overthrow them. The Soviet state tried a number of different things to destroy Orthodox Christianity, including setting up a modernist church that had the form but not the substance of Orthodoxy, and sometimes just plain old persecuting anyone who admitted to being Orthodox. Children, for example, were not allowed to attend Church. Sermons were supposed to be cleared with the local authorities before they could be delivered. Being Orthodox was excuse enough to run you into the Gulag and probably kill you (actually, according to what is said in The Gulag Archipelago by Solzhenitsyn, just about any reason was good enough to throw you in the Gulag). In reality, Sergius' declaration had some effect, but not nearly as much as he had hoped (or was led to believe that it would have). It wasn't until the 2nd World War that the Soviet government lightened up considerably on Orthodoxy, and then only because, seeing that 3 decades of persecution had not destroyed the Church, it changed tactics and used the Church even more than it had before as a tool to further it's own aims.
I can't imagine what it would have been like to live in those times. I don't personally call Sergius a heretic. Not anymore anyway. That's not my call to make. I am sorry that there are still similar situations going on today as well.
 Fwiw, the introduction of the first book on Fr. Arseny had this to say about the Soviet government's treatment of Christians:
"...it has been revealed that six hundred bishops, forty thousand priests, and one hundred twenty thousand monks and nuns were killed during this period. Many of these died in the harsh conditions of prison or labor camp; others were shot or buried alive. By the end of Stalin's dictatorship, only some two hundred priests remained active in the Soviet Union. The scale of this martyrdom is unprecedented in the history of the Christian Church.
Those who attended church services were watched carefully; they often lost their jobs and other opportunities--their children could be refused entry to universities... religious education of any kind at home, in church, or at school was strictly forbidden, as was all religious literature." - Father Arseny: 1893-1973; Priest, Prisoner, Spiritual Father, (St. Vladimir's Seminary Press, 2001), pp. vi-vii
