He was born in
Beveren (Alveringem), the son of a Flemish preacher Pieter Michielszoon Bardt, who left Flanders for religious exile in London around 1568, with his family. In 1577 Petrus Bertius returned to the Netherlands, to study at the
University of Leiden. He supported himself by tutoring younger students and continued travelling in Europe. In 1593 he was appointed to subregent of the Leiden
Statencollege, marrying in the same year Maritgen, daughter of
Johannes Kuchlinus, the first regent of the
Statencollege, whom he would succeed after his death in 1606 as a regent. He was also connected by marriage to
Jodocus Hondius and
Pieter van den Keere, his brothers-in-law and both cartographers, and this would influence his later life. ==Philosophy & theology==