Already in the 15th century the University of Cracow had brought forth humanist scholars of European repute, and begun to attract distinguished lecturers and numerous students from abroad. Owing to Sigismund I.'s marriage to a Sforza of Milan, the royal court at Cracow became the home of the highest Renaissance art of Italy, and the royal castle on Wawel hill at Cracow, rebuilt by Italian architects and their Polish disciples, became one of the finest monuments of Renaissance style north of the Alps. Under Sigismund II., the third great spiritual factor of the age, next to humanist scholarship and Renaissance art— the doctrine of the Reformation—entered potently into Poland's intellectual life, uniting with Italian culture on the common ground of literature, and helping to produce the first great age of Polish poetry and prose. At the same time, the clash between the New Learning and the strong tradition of Poland's chivalrous Catholicism; the difficulties with the Scandinavian Powers and the rising empire of the Moscow tsars; the dilemma produced between the evolution of the Polish Parliamentary system, and the Renaissance tendency towards the strengthening of central Government authority : even a king of genius could be only partially successful in coping with all these tasks, and the reign of Sigismund II., in many respects one of the most brilliant in Polish annals, left the seeds of decay and failure behind it.
renewed by another diet in 1555, during which Masses were actually said in Polish, and the Communion was administered in two kinds. A religious interim of about ten years followed, during which Protestantism in Poland flourished exceedingly. Presently reformers of every shade of opinion, even those who were toler ated nowhere else, poured into Poland, which speedily became the battle-ground of all the sects of Europe. Soon the Protestants became numerous enough to form ecclesiastical districts of their own. The first Calvinist synod in Poland was held at PilaczOw in 1550. The Bohemian Brethren, expelled from their own country, ultimately coalesced with the Calvinist at the synod of Kozminek (Aug. 1555). In the diet itself the Protestants were absolutely supreme, and invariably elected a Calvinist to be their marshal. The king, however, perceiving a danger to the constitution in the violence of the gentry, not only supported the bishops, but quashed reiterated demands for a national synod. The diet of 1558-59 indicates the high-water mark of Polish Protestantism. From this time forward it began to subside, gradually but un mistakably, chiefly owing to the division among the reformers themselves. From the chaos of creeds resulted a chaos of ideas on all imaginable subjects, politics included. The Anti-Trinitarian heresy proved to be the chief dissolvent, and from 156o onwards the relations between the Lutherans and the Calvinists, were fratricidal rather than fraternal: Jan Laski (q.v.), vainly strove to unite all Polish Protestants round the Helvetian standard ; and a Federation of all Poles of the reformed faith—the "Concord of Sandomierz," 1570,—being predominantly Calvinist in character, met resolute Lutheran opposition and led to nothing.
While the strong individualism of the Polish national character thus thwarted all endeavours at Protestant consolidation, the wars against Orthodox Moscow, effectively united Poles round their old Catholic banners; and the vigorous Protestant propaganda con ducted from Konigsberg by Poland's vassal, Prince Albert of East Prussia, appeared to the bulk of the nation under the guise of a German menace. These political factors told in favour of Catholicism : so did presently the wiser policy of Rome. Pope Pius IV., unlike his predecessor, adopted a conciliatory attitude towards the Polish Crown in the matter of disputed appointments of bishops. The new bishops were holy and learned men, very un like the creations of Queen Bona Sforza, and capable papal nun cios reorganized the scattered and faint-hearted Catholic forces in the land. From one of the ablest of them, Giovanni Commen done, the king, at the diet of 1564, accepted the book of the decrees of the Council of Trent, and immediately afterwards issued decrees banishing the more extreme heretics from the country. In 1565, the Jesuits, the vanguard of the Catholic Counter-Reformation appeared in Poland.