Stalin's position as a true believer is dubious at best.
I mean he certainly didn't believe in democracy, but the way the economy of the USSR was run was the most communist thing the world had ever seen by that point (that lasted longer than three years). The communism s mixed with Stalin's authoritarianism, bigotry and paranoia, but there's no way collectivization and the five year plans were anything but (preparation for) communism. Besides, Stalin was a Bolshevik from when they were robbing banks to fund their activities, so it's not like he joined for the upward mobility. If he wasn't a true believer he could've been doing literally anything else with his life.