Stalin was raised Eastern Orthodox Catholic, and openly became atheistic in his views.
It became highly dangerous to publicly profess or practice religion under his rule, and most religious institutions were eradicated at his word.
He enacted purges of all religions and had strict anti-religious laws in place. He taught anti-religious propaganda in schools, and atheistic education.
The important point that many will most likely miss is the difference between correlation and causation.
Stalin, Genghis Khan, Vlad the Impaler, Saddam, Hitler, and Augusto Pinochet might have all had an absence of belief in the Loch Ness monster as well.
It would be amusing for someone to try and make the claim that because of this absence of belief in an aquatic monster residing somewhere in the Scottish Highlands, they decided to kill a whole mess of people because of it.
There is actually observable, rigorous criteria to being Christian (because it's a lifestyle). There is no criteria to be atheist being it's a secondary state (a belief claim).
This is like asking "Was Stalin, a self-proclaimed human, really human?" and comparing it to, "Was Ghandi, a self-proclaimed supporter of non-violence and pacifism, really a pacifist?"
I can't believe you fell to this low.
Stalin said he was an atheist and so we should recognize him as such. Just as Tim McVeigh called himself a Christian and the 9/11 hijackers called themselves Muslims. I do not approve of what he did, but I will agree that he was an atheist. I am gathering that most atheists will agree with this poll, or at least I hope so.
First and foremost, Stalin was a shrewd politician.
He saw an opportunity to gather political support for his rule, by attacking the corruption of the Patriarchy of the Orthodox Church.
The Russian people greatly supported this.
Following his rise to power, Stalin reinstated the Orthodox Church, and was a tremendous proponent of the teachings.
So what Stalin might have thought, and how he behaved, are two very different things.
There really isn't any evidence to suggest he was a true Atheist.
Stalin often made notes in the books he read. One such note indicates he was a spiritual individual.
Stalin left such mark about God in the book by Anatole France "Last pages. Dialogues under a rose. About God": "Don‘t know traces, don‘t see. There is no Him for them." ("Следов не знают, не видят. Его для них нет."). It seems that Stalin thought he knew God’s traces and saw God, not like others.
Stalin actively criticized atheist writings.
Stalin "denied categorically to prescribe atheistic literature to his personal library, fastidiously calling it ""antireligious waste-paper (junk)"
Stalin was a poet (believe it or not) who wrote in his native Georgian language, quite often referring to god and faith.
(...) In his burden and song like the beam of the Sun, lived great truth – DIVINE dream.
(...) But people who forgot GOD, with darkness in their hearts, instead of wine served poison to him in the cup.
And told to him: "Damn you! Drink this cup to the bottom!... And your song is alien for us, and we don’t need your truth!"
The only real quote Stalin has ever reported making in favor of atheism was written by a man under the Khrushchev years. And Nikita Khrushchev was a militant atheist, who needed to capitalize on Stalinist support to solidify his power, so it is quite possible he had attributed to Stalin words which Stalin did not say.