He was ordained deacon and then, at Shanghai in August 1845, priest, by the French bishop Jean-Joseph Ferreol, whom he then brought into Korea by sea in a small boat with a lay companion. He was twenty-four. His work over the following year was practical rather than pastoral in the ordinary sense: identifying landing places, arranging contact with Chinese fishing fleets off the Yellow Sea coast, and drawing charts that would let missionaries enter without crossing the guarded northern border.
He was arrested in June 1846 while attempting exactly this, interrogated, and beheaded at Saenamteo on the bank of the Han river in September of that year. The letters he wrote from prison to his congregation and to Bishop Ferreol survive, and they are the reason the record of his last months is unusually full.
He was canonised in Seoul in May 1984 by John Paul II, in a group of one hundred and three Korean martyrs drawn from the persecutions between 1839 and 1867. Because they were canonised together, the group is commemorated on a single feast day under his name and that of Paul Chong Hasang, a lay leader executed in 1839. Korean Catholicism is today one of the larger Christian communities in Asia, and Saenamteo is a shrine.