Let's get this out of the way up front - my opinions about Stalin:

Stalin was one of the the most effective leaders any country has ever had. He was instrumental in turning Russia into the Soviet juggernaut that, for a time, was equaled only by the United States in international power and influence, he was a key figure in removing a power structure from his country that had held terrible sway for centuries and had largely reduced it to squalor, he was able to do the most important thing (re: leave the biggest decisions in the hands of his generals) when it came time for the key initiatives to be played-out on the Eastern Front in WWII and managed his country's war economy so effectively that even after the Germans had made their deepest penetrations into Soviet holdings, the production of T-34s was such that the Soviet tanks outnumbered German Panzer IVs and Panthers by odds of 3 to 1 even in the darkest days of the war.

Stalin's rule was brutal, egotistical and, in many places, outright disgusting. I find it notable, however, how very 'pro-Soviet' and 'pro-Stalin' most of the Soviet public remained, even in the shadow of their dictator. He was VERY GOOD at what he did, and even better at making people see this fact.

So, that being said:

Stalin, almost unquestionably, did not himself buy into superstition - including the notion of an all-powerful Abrahamic God. He did not respect the divine right of Nicholas II (who would be Russia's last tsar), did not believe in (and did much to discredit) the psychic and healing powers of Grigori Rasputin and, while he was in power, mercilessly prosecuted organized religion.

Stalin was a nihilist, and 'collectivism' became his own religion. Even if the word 'God' never touched his lips, Stalin very much saw himself as a divine force amidst so much rabble.

The first benchmark for Stalin's rise to power came following his expulsion from school, where he read the works of Vladimir Lenin - inspiring him to become a revolutionary. Stalin's revolutionary activities, including the organization of strikes and making public speeches, fairly quickly lead him to the acquaintanceship - and then undying loyalty - of Simon Ter-Petrossian (the infamous 'Kamo'), a violent psychopath whom would prove a key piece of Stalin's early network as his personal monster and guard dog.

The first really major (and sadistic) play that Stalin would make involved very careful manipulation of an arson (which he was likely part of) at a Batumi oil refinery. After the fire, workers expected to be compensated with bonus pay for their assistance in putting out the flames. The refinery management refused, suspecting the arson. This allowed Stalin to easily persuade the works into striking, and the strike spiralled into clashes with the Cossacks. After a series of arrests were made, some enterprising workers attempted to break their friends out of jail - and thirteen were killed in the ensuing struggle. This was precisely the result Stalin had wanted:

He lofted photos of the dead, hailing them as matryrs, which ingnited passions.

Stalin was arrested and exiled to Siberia for three years, the secret police finally catching-up with him. This was a fateful mistake - while in Siberia, Stalin learned even more about the revolutionist movement, including the fact that there were two 'parties' involved, and one of those parties was the Leninist 'Bolsheviks'. Stalin immediately became a Bolshevik himself, and having only spent ten days of a 3-year sentence in exile, snuck back into the heart of Russia.

The war between Japan and Russia broke-out, leaving Russia in fiscal ruin and straining it's base of military forces loyal to the tsar. Restlessness swept across the country, and Stalin's exploits in fuelling the fire of the general malcontent finally brought him to the attention of Lenin.

The unrest culminated in the killing of 200 demonstrators by Cossacks at a mass demonstration. This sparked the first Russian Revolition, which created all of the conditions and reforms necessary for the second Russian Revolution (or October Revolution) in which the tsars were overthrown (Stalin spent the interim period with a gang and his personal goon, Kamo, robbing banks in order to aquire the holy trinity of notoriety, talent and money).

Following the revolution and ensuing civil war, and after Lenin's death (which prompted Stalin to make a very religious pledge to his dead former mentor and leader), Stalin used what can only be described as absolute cunning in order to politically defeat his fellow contenders for the leadership of the nation. He put himself on the majority side of every battle, slowly whittling down his opponents one by one, until he stood as the prime mover of Russia.

After the Great Purge, with the secret service being escalated into a position of terrifying reach, Stalin emerged the absolute ruler of the new Soviet Union.

This was a tough one to encapsulate. The Russian revolution and revolutionaries were very complex, and unlike (say) Hitler's rise to power, Stalin's was far less certain. There were many contenders that very realistically 'could have' taken the country's reigns at the end - Stalin just happened to have been the one who wound-up with them (largely because he was not only intelligent, but extremely dangerous and violent). Stalin's Credo is also not the same kind of article that, say, Mein Kampf is, in that it's not an introspective journal. Stalin intended for it to reflect positively on himself (and had a very good grasp of how to do that), and it's riddled with thoughts the man wanted people to think of him - not what his actual thoughts were.

In any case, once again, we don't see Darwinism as the vehicle here. Certainly, Stalin was opposed to organized religion and made no time in his life for superstition - but he was deeply religious regarding his self-image and the power of communism. His actual deeds were fueled by his ambitions, eagerness for bloodshed and his use of violent people as his favored tools.