Stalin was fundamentally an ideologically motivated pragmatist, and ruthlessly so. There was always a core ideology at work as a set of goals and he worked with the means available to achieve them. He originally took power by supporting market based economic reforms, only to then dramatically shift towards forced collectivisation, state ownership and rapid industrialisation in the thirties. His belief that Nazism was a form of Capitalist Reaction did not prevent him from agreeing to the Nazi-Soviet pact and dividing up Poland to his advantage. Nor was his hostility to British and American Imperialism a reason to reject the assistance of the Western Allies in world war II.
Whilst Stalin defeated Trotsky by advancing the theory of "Socialism in one country" it was more a question of emphasis rather than of rejecting world revolution and internationalism as a whole. He was never a "nationalist" in the sense we would typically understand it because the Marxist conception of nation was based on class. He presided over a Federal Union of Republics and introduced policies similar to affirmative action to ensure "national self-determination" of various ethnic groups within the USSR, whilst also committing genocide against some groups that were considered "disloyal".
It is true that Stalin did make concessions to the Christian Churches, but these were primarily tactical considerations rather than rejecting the goal of eliminating religion as a superstition that consolidated the power of exploiting classes over the masses. These concessions were also after a decade of the most vicious and uncompromising assault on religious ideas and institutions history had then known. The Soviets persecuted all religions without qualification, including Jews, Muslims, Christians, indigenous beliefs of nomadic tribes in Siberia, etc.
Stalin is an extremely complex and controversial figure but I have no doubt about the sincerity of his convictions as a Marxist and a materialist. It is in fact difficult to comprehend how he could have even been capable of behaving in the way he did without a depth of conviction and an uncompromising commitment to those convictions that could make the deaths of millions, including some of his friends and family members, as well as risking his own life for a cause as an acceptable price to pay for achieving his goal. It is a quality that deserves respect and fear for how it shows mankind's ability to justify its worst vices in pursuing its virtues. Communism was a goal in which man had no masters and God is nothing but an enemy of the people as an obstacle to their power and freedom. I don't know the man obviously, but for him to be a Christian would seem to be a betrayal of his life's work to build a rational, scientific order in which mankind triumphs over the blind operation of natural and social forces, "God" being only one of them.
