Why are countries like Estonia, The Czech Republic, and Albania so much more irreligious than countries like Poland, Romania, Slovakia, and Lithuania? These countries are certainly very different, but perhaps someone here has enough knowledge of at least two of these countries to give a good explanation.
I find it especially interesting that Estonia and Lithuania, once both part of the Soviet Union, are very different today in terms of religious belief.
I'm familiar with both Albania and Poland's religious situation, so I'll comment about them.
Albania has never been a very religious culture. Back when Islam was introduced during the 14th century, Albanians took it up in large numbers. But they largely didn't do so for spiritual reasons. It was mainly a pragmatic decision.
As Albania was incorporated into the Ottoman Empire, the Albanian people wanted to maintain their traditional way of life, which included keeping arms. However the Ottoman did not permit non-Muslims to do so, fearing they would revolt. Thus the Albanians converted to enjoy the benefit of maintaining arms, though they were far from devout followers. Many also professed to follow Christianity (either Catholicism or Orthodoxy; both had a presence), and even utilised both Christian and Muslim names.
Poland is quite different. Poles have long placed importance on their Catholic beliefs, and consider it an important aspect of Polish culture. Even during the Communist era it was impossible to fully eradicate religion from Poland. This is evident by the celebrations in 1966 commemorating 1000 years since Poland was baptised; though tolerated by the authorities, they placed emphasis on the date as the establishment of the first Polish state, though the two events are intertwined.
The opposition movement also had strong church backing, and one of the most important figures throughout this era was Stefan Wyszynski, who was Archbishop of Warsaw from 1948 to 1981. He took an active role in opposing Communism, and helped launch the millennial celebrations in 1966. So by uniting defiance to Communism with religion and nationalism, Christianity in Poland maintained a strong grasp, which continues, albeit lesser than before.
Unfortunately I'm not familiar enough with the Baltics, let alone their stance on religion, to add anything on them. But hopefully these two examples help provide some context.
Estonians were moderately religious Christians during the interwar period during the 19th century. However, it was really a mixed bag of Lutherans, Baptists and Orthodox, along with a number of smaller confessions. Religion was not a central part of the national identity, however, and I'd say Soviet repressions against religion were more effective here.
Lithuania, on the other hand, historically had close ties to Poland (see: Polish-Lithuanian Commonwealth), and Catholicism was an important part of their historical narrative. Therefore, they resisted anti-religious propaganda and repressions.
Admittedly, being an Estonian Baptist, it's quite likely my interpretation is biased.