Excommunication was an act of religious punishment in which a person was deprived of the privileges of their religion. Depending on the degree of integration of church and state, it could also be a grave political act — since it would force the excommunicant outside of the institutionalised religion, making it impossible for them to fulfil their sworn duties.
In early 17th century France, the church and the state were intertwined to a particularly profound extent, since Cardinal Richelieu — as he called himself, "the Pope's representative in France" — was also King Louis XIII's prime minister. When Richelieu's political goals were opposed by the king, he used his role as cardinal to threaten Louis with excommunication from the Roman Catholic Church. Had he been successful, this would have at least theoretically made it impossible for Louis to fulfil his coronation oath. In the end, however, Louis simply had Richelieu jailed and thus prevented the necessary communication with Rome. (AUDIO: The Church and the Crown)