His 1553 "Des merveilles du monde et principalemẽt des admirables choses des Indes & du noveau monde" is one of the earliest European descriptions of religion in Japan. He interprets Japanese religion in terms of his universalist views on religion, claiming that the indigenous Japanese religion was a form of Christianity and that one could still find evidence of their worship of crucifixes. Such claims about Japanese religion were common in Europe at the time; Postel's writings may have influenced Francis Xavier's expectations of Japan as he traveled there.