Complex sentence: The conversion to the Roman Catholic faith (and to a lesser extent, Protestant) was often the single most important part of the process as for Ruthenians of that time being Polish culturally and Roman Catholic by religion was almost the same.
Split into simpler sentences: The conversion to the Roman Catholic (and to a lesser extent, Protestant) faith was often the single most important part of the process. For Ruthenians of that time, being Polish culturally and Roman Catholic by religion was almost the same.