 Despite his upbringing, the young Princen conceived an interest in Catholicism under the influence of the parents of his mother, Theresia Princen-Van der Lee.In 1939 he entered the Holy Ghost Seminary at Weert – where he was followed by his younger brother Kees Princen, with whom he was to maintain correspondence throughout all the vicissitudes of his life.It was while he was at the seminary that Nazi Germany invaded and occupied the Netherlands in 1940.