From A Concise History of Romania (Cambridge Concise Histories), by Keith Hitchins (Cambridge, 2014), Kindle Loc. 1018-1031:
The Habsburgs in Transylvania were confronted by institutions and mentalities that slowed the absorption of the principality into the general structures of the empire. To succeed, then, they would have to undermine those autonomies that had arisen since the era of settlement by the Hungarians, Saxons, and Szeklers and had taken form in the so-called Union of the Three Nations in 1438. The Union evolved into a monopoly of power and privilege imposed by the Hungarian nobility, the Saxon urban patriciate, and the upper classes among the Szeklers. They were the three nations. Social class, not ethnicity, determined membership, and, thus, the masses of Hungarian, Saxon, and Szekler peasants and others were excluded. The three nations in the fifteenth century were, naturally, Roman Catholic, but in the sixteenth century the Protestant Reformation made many converts among the Hungarians (Calvinist and Unitarian), Saxons (Lutheran), and Szeklers (Calvinist). The new Protestants and the remaining Roman Catholics eventually reached an understanding, and adherence to one of their churches became a condition of political privilege, that is, of membership in one of the nations. The three nations and four churches formed the backbone of Transylvania’s autonomy when the Habsburgs arrived. The Romanians, who composed perhaps half the population of Transylvania in the early eighteenth century, were not a part of this system. They were excluded because they were Orthodox and overwhelmingly peasant.
During these centuries the Romanian Orthodox Church had led a precarious existence as merely tolerated by the three nations, but had, nonetheless, been able to maintain an administrative organization and a hierarchy presided over by a Metropolitan in the sixteenth and seventeenth centuries. In the seventeenth century the church was subject to heavy pressure from the Calvinist princes who were determined to convert the Orthodox clergy and faithful to Calvinism.