ENThe present article deals with the decision of the Russian government to introduce the Russian language in the supplementary services of the Roman Catholic Church in the Western Territory after the suppression of the uprising of 1863-64. The analysis of the genesis of this idea reveals that this decision was passed after long discussions in which the arguments of Russian officials and publicists were based on different concepts of nationality (in that context nationality meant the distribution of the subjects of the empire into social groups according to political and/or ethno-cultural categories). The proponents of the introduction considered the language the principal criterion of nationality. Their opponents maintained that Orthodoxy was the best guarantee of Russia’s integrity and the main implement of assimilation. Meanwhile, the Catholic faith meant associations with Poland, no matter in what language the religious services were conducted. The emphasis on Orthodoxy implied linguistic assimilation, at least as regards religion. However, this was not the general trend in the official reasoning: after the uprising Nikolai Miliutin supported the idea of translating the Orthodox liturgy into Lithuanian.A gradual enforcement of the Russian language on the Catholic Church after the uprising signified a change in the conception of nationality, to be more precise - a shift in priorities, on the part of the Russian authorities. The ban of 1848 to use the Russian language in the Catholic services meant that Orthodoxy was treated as the main constituent of Russian nationality. Meanwhile the introduction of Russian in the education of children in the faith (an exception was made for the Lithuanians of the Kaunas Province) and afterwards a permission given in 1869 to use that language in the supplementary services proved that Russian was recognized as the principal implement of assimilation and a fundamental attribute of Russianness. In future it was ‘to return Russian Catholics’ to Orthodoxy. [From the publication]