Reformation in Eastern Europe
Poland
Poland may be thoroughly Catholic today, but the Reformation made great progress in that country in the middle and later 1500s. It is a prime example of the weaknesses of the reforming movement and the successes of the Catholic Reformation.
Students from Wittenberg brought the reforming message early to Danzig and Cracow, but here as elsewhere national sentiments directly affected the course of the reform movement. Partly because of Polish political traditions, partly because of long-standing ties with France, and partly because of a growing antipathy toward Germans, the Poles took much more strongly to Calvinism and Calvinistic sects. It's worth pointing out, too, that the Hussites had flourished in western Poland, so the country had a long tradition of dissatisfaction with the clergy. Moreover, the country also had a long tradition of religious toleration: many Jews had fled thither from persecutions in the West, and there was even an Islamic Tatar population in Lithuania. In the event, a number of different reform churches took root in Poland, especially during the 1540s and 1550s. While ideas and enclaves could be found everywhere, different flavors of Protestantism flourished in different regions of Poland, for exactly the same reason they did in Germany: due to the preferences and protection of the local nobility.
The 16th century Polish kings were either indifferent to the reform issues, or genuinely believed it was not the place of a king to interfere in religious disputes. In any event, the Polish king could do little effectively without the cooperation of the Polish Diet (Sjem), and the Diet was dominated by reformist princes. Indeed, in the 1550s, they were strong enough to get legislation passed specifically excluding the Catholic Church from a variety of public spheres. Poland seemed to be travelling down the same road as other Protestant nations.
But it never got much further. the reasons are complex and make for a fascinating study. To oversimplify here, I will boil it down to two main factors: the variety of Protestants sects within Poland, and a renewed Catholic vigor.
Calvinists were perhaps strongest in Poland, but there were also a number of Zwinglians. Besides these, a strong group of Socinians (Anti-Trinitarians) were protected by a variety of princes. There was even a kind of re-birth of the Hussites, known as the Bohemian Brotherhood. While these (and other) groups could agree on a handful of core principles, when attempts at reform went further, their differences weakened them in the face of Catholic opposition. Even their great proclamation of 1570, the Concord of Sandomir, was boycotted by the Socinians.
Beyond these divisions, the poor of Poland never abandoned the Catholic cause. With the nobility going Protestant, the peasants and lesser nobility, who were steadfastly in opposition to the great nobles and viewed the king as their ally, naturally took the opposite view in religion. Thus, even where the Protestants were strong, a significant portion of the population remained Catholic.
This provided fertile ground for the post-Tridentine Church (that is, the Catholic Church after the Council of Trent). Led by Bishop Hosius of Warmia, and by the papal legate John Francis Commendoni, the Catholic Church in Poland made a determined effort to reform itself and to win back those who had fallen away. Here there was no Inquisition nor decisions on the field of battle. The Counter-Reformation in Poland was generally peaceful and successful. The Hosius and Commendoni brought Jesuits into the country, and the Order founded schools, debated the Protestants, and helped greatly to restore the credibility and respectability of the Catholic Church.
Interestingly, the Protestants were not run out of the country. Rather, they continued to be tolerated by the government. But throughout Europe, the Protestant cause only flourished when it was able to associate itself with the national identity, and that it failed to do in Poland. Protestants simply became fewer and fewer in number, until they were only a tiny minority.
A postscript is worth mentioning here. Nicholas Copernicus died in 1543. He spent most of his career in Poland as a practicing medical doctor but deriving his steady income as a canon in the Church at Warmia (Ermland). The Protestant Reformation was in full swing at the time, and was just really catching fire in Poland itself. Given that political-religious environment, and given his own position as a Church employee, it's hardly surprising that Copernicus chose not to publish his On the Revolutions of the Heavenly Spheres until after his death.
Reformation in Hungary
The Battle of Mohács was fought in 1526. This was a major victory for the Turks and resulted in a partition of the kingdom: so-called Royal Hungary, which was in the west, Transylvania, and the rest (over half the original kingdom, including Buda and Pest) was taken by the Ottoman Turks. The spread of religious reform was conditioned by the over-arching political reality of a truncated and divided kingdom facing a powerful and active enemy on its immediate borders.
In this context, the Reformation spread rapidly. Central power was too weak to respond, the call to reform was powerful, and the barons of Hungary had every reason to like the idea of churches under their more direct control. In any case, both Ferdinand I and Maximilian II were inclined to be tolerant of dissent. Hungary became home to some fairly radical sects, including a strong contingent of Antitrinitarians.
In Royal Hungary, the Lutheran and the Reformed churches prospered, with the Helvetian Confession being formally adopted in 1567 and the Lutheran confession adopted in 1545. More or less, the division was ethnic, with the Germans in Hungary adopting Lutheranism and the Magyars adopting the Helvetic Confission (Zwinglianism). Gaspar Karoly published a Hungarian Bible in 1590. The Vizsoly Bible was published in 1607 and is sort of the King James Bible for the Hungarian Reformed Church.
Socinianism (Antitrinitarianism) found a home in Transylvania. It persisted despite widespread persecution and can still be found there today.
The Counter-Reformation hit Hungary hard. By 1600 nearly everyone (perhaps 80% to 90%) in Hungary was Protestant. Its ruler, however, was Ferdinand II and he was determined to eliminate all except the Roman Catholic Church. He went after the ruling families, persuading them by one means or another to convert to Catholicism and expecting everyone within their lands to convert with them. On a practical level this meant that the noble baron would no longer pay Protestant preachers, would support only Catholic churches, would support and promote monasteries and Catholic evangelical efforts under the Jesuits, and would even formally ban open practice of non-Catholic rites.
In 1673 those Protestant ministers who were still resisting were summoned to a court hearing. Thirty-three responded and were either pardoned or else allowed to resign their ministry. Those who did not respond were arrested and were sold as galley slaves. It caused quite a scandal and they were at length set free by Dutch shipmasters.