"ticket-of-leave apostle"

A ticket-of-leave was a discharge given to a convict or soldier, which carried certain conditions – usually, in the case of a convict, it required him to remain in a certain area and follow certain occupations. As Michaelis was imprisoned for life and then paroled, the ticket-of-leave could well be taken in this sense. However, Conrad allows for a double-meaning by throwing in the word apostle, implying that Michaelis is on leave from saintly duties, an idea reinforced by other descriptions of him throughout the book.