This member of the house of Vítek was a significant personage of his time. Through marriage with Viola Těšínská, the widow of Wenceslas III, he entered into kinship with the royal family and thus gained prominence, which allowed a confidential relation with the Queen Elisabeth Rejčka to another Bohemian magnate, Henry of Lipé. This already predestined him for an outstanding position, not only in the Vítek family but also in the whole country. Peter was a supporter of King John of Luxemburg, who issued a paper in 1336 for him and William of Landstein, which provided both magnates with a "privilege from all inhabitants of the Bohemian country, lords, noblemen, and other persons of any status," and appointed them "servants peculiar and closest to him".
John of Luxemburg's son, Charles, then yet a young margrave, used Peter's services no less than his father. The ambitious Rosenberg attained the high degree of the highest chamberlain very early, in 1323, and moreover, Charles appointed him the head of the Provincial High Court. It appears that when he journeyed to Lithuania with his son, the king left Peter in charge of the whole kingdom. So it is not surprising that four years later, King John took him, alone with only other two magnates, into the closest circle of his administrators. The successor to the throne, Charles showed the same confidence in Peter in 1339 when he granted him the administration of a district in Bohemia for the period of his own absence. This meant nothing less than that this Rosenberg became a deputy ruler and the second highest person in the country for a certain time. Peter of Rosenberg was mortally ill at the time of the coronation of Charles IV as king of Bohemia in 1347 and he died soon afterwards, on 15th October. Not long before his death, he took refuge for a last stay in his family monastery and accepted the robes of the Cistercian order on his deathbed. It was in this monastery that he was buried.
It seems that Peter I had considerable merits in the monastery in Vyšší Brod. In 1346, he donated a large amount of money for the construction of the cloister, and one year later he founded a monastery hospital. Notes about donations that he made to the monastery can be traced back to 1332. The position of Peter of Rosenberg, his importance in Bohemia and among the representatives of the House of Vítek, and his relationship to the monastery in Vyšší Brod, all indicate that it was he who ordered the altar that is now known as the Vyšší Brod altar for the abbey church, apparently not long before his death. This would seem likely even if there were not a figure of the donor with his symbol on the painting. It is certain that the picture called "The Birth of Christ" shows the kneeling donor with a model of the church in his hands and with the sign of the cinquefoil red rose in a slanted coat of arms in the lower right corner. The donor had himself depicted on the above-mentioned panel in particularly ostentatious clothes in accordance with his position and ambitions. He is wearing a duchy hat hinged on his back, and he is mantled in a precious ermine coat. In the Middle Ages, the wearing of ermine fur was a privilege of the members of the ruling class and of the mundane and spiritual aristocracy. Charles IV and his son appear in the illuminated manuscript of the Golden Papal Bull in this manner as does Elisabeth Pomořanská in the wall paintings of the St. Wenceslas Chapel in St. Vitus cathedral. The doges of Venice also had themselves depicted in this way. Even these costume details demonstrate the self-confident character of the donor, and to some extent they belong to the sovereign pathos of his commission as well.
Undoubtedly, Peter of Rosenberg ordered the altar in Prague from a painter active directly in the courtly circle and with his workshop closely associated with it. This is just another manifestation of the traditional rivalry between the Víteks and the Prague royal family. After all, the Vyšší Brod monastery itself is known to have been built because of the rivalry between the Víteks and Premyslides. It served as the dynastic burial-ground for the Víteks just as the Cistercian abbey church at Zbraslav served for the Premyslides.