The history of the Greek Catholic Church, which first defined a Greek Rite within Western Christianity, is bound up with the emergence of Lithuania, later merged into Poland-Lithuania, as a Catholic state that conquered the predominantly Orthodox lands of the Rus' in Eastern Europe. What was historically called the Uniate Church was set up to accommodate the local Christians and their ecclesiastic leadership under the Catholic umbrella in a state known for its religious tolerance. At the time, the religious boundaries of the Schism were comparatively fluid, and the leadership of what is now western Ukraine had from the 13th to the 15th centuries repeatedly vacillated between eastern and western leadership. The Union of Brest in 1595 finalized the shift of the Orthodox leadership of the lands of White and Little Russia (modern Belarus and Ukraine) to Uniate status. The population of those countries became Greek Catholic without a break in administration. Later, when Muscovite Russia conquered the same, the ecclesiastical leadership largely switched its allegiance again. The modern Ukrainian, Ruthenian, and Hungarian Greek Catholic Churches (approx. 5 million total) compose the great majority of Greek Catholics today, but are only a fraction of the early modern Greek Catholic or Uniate population.