Some points about Polish religiousness as I see it:
- the Pole=Roman Catholic identification (as we sometimes put it, the "polakatolik" identity) began forming as the outcome of the numerous wars waged by the Commonwealth in the 17th century against Protestant Swedes, Prussians and Transylvanians, Orthodox and Greek Catholic Cossacks and Muscovites as well as Muslim Turks; as a result it was easy for the Catholic majority to blame religious minorities for inviting and siding with the invaders; the previous religious tolerance started to erode (especially
the Polish Brethren were forced to emigrate or convert as a scapegoat for the disasters that had happened) and the Catholic Church gradually became a "national totem" serving to differentiate oneself from other ethnicities. Today this Catholic flaunting is particularly visible in eastern Poland and the Polish communities beyond our eastern border (not that Poles don't build all the
kitschy Licheń's and forty meter crosses and
Jesus statues elsewhere but the Church is most all-pervading and alternative-less in the east, I think)
- in Czechia after the Hussites were crushed Catholicism came to be seen as a religion superimposed from outside and thus unpopular; in Poland there was no comparable development - the spread of Protestantism was halted because it was seen as fostered by external forces (see above)
On why religiosity didn't collapse in the 20th century under communism as in other countries:
- strong Catholic basis to begin with, the
polakatolik stereotype was already formed
- it has to be stressed that the communism in Poland lasted three decades shorter than in the USSR and came after the initial revolutionary zeal and the 30's Stalinist craze of purges; it was also far less extreme in its policies e.g. regarding the collectivisation of agriculture; atrocities were rampant especially in the initial period when Stalin was still alive, however we didn't have gulags and holodomor (but court murders, short-lived-yet-vile concentration camps and food shortage riots - yes); communist ideology was also perceived by many as foreign, imposed by Russians, which strengthened the attractiveness of the Church as a strong institution that was covertly opposed to the state, a gathering place for those discontented with it
- Roman Catholicism was probably harder to infiltrate than say Orthodoxy as it was governed from outside and didn't have the tradition of submission to the czar (from what I've heard, Russian Orthodoxy was practically recast into a ministry executing the czar's will under Peter I)
- Polish upper classes were decimated in the course of the century in events such as the Nazi German massacres of intelligentsia, war-period Soviet deportations and mass murders of Polish officers, the Warsaw uprising, the communist clampdown on Polish war-period (and anti-communist) guerrillas etc. Cities were repopulated with peasants with their rather more traditional and religious outlook on life