He came from Uppland and was a son of the important high noble family of Rumby, being son of Sir Filip Finvidson of Rumby, called Filip Röde, and his wife, herself the daughter of Lord Karl Tjelveson of Fånö and his wife from the Lejonörn family. His relatives and forefathers were important magnates in Uppland, and they also are said to have descended from royal stock. His close forefathers and relatives had apparently in the previous century been the nucleus of the so-called Folkunge Party, several times in rebellion against kings centralizing government. Petrus' one uncle (John of Fånö) had been executed in 1280 and his own father, Sir Filip, only just spared his life, by paying immense monetary penalties. On the other hand, the family had then allied with central government, for example one of his sisters (Ingegerd Filipsdotter) married king Magnus III's illegitimate nephew.
Petrus had only a brief-period ecclesiastical career before he was elected Archbishop. Following his election, he travelled to Avignon, where the Pope John XXII was residing, to get ordained into the episcopal office.
He belonged to a monastic order because in 1337, he was ineligible to inherit secular property, as was Swedish law of that time, distinguishing between monks and priests (the latter were eligible to inherit).