During the first half of the seventeenth century debate over the Copernican hypothesis had spread beyond the ranks of astronomers and had stirred up so much controversy that the Church decided to intervene. In 1616 a theological examining body concluded that the idea of the earth's motion was philosophically false and in conflict with the Scriptures, and it suspended Copernicus's book until corrected. Historians have generally assumed that this decision and the subsequent condemnation of Galileo had such a devastating effect that scientific progress in Catholic countries was greatly retarded. However, the attitude of Marin Mersenne (1588-1648), who was both a faithful member of a religious order and a central figure in the development of French science, does not support such a conclusion. An examination of Mersenne's reaction to Copernicanism indicates that no matter how disturbing the Church's decision, it was still possible, at least in France, to study Copernican ideas and to find them useful, despite some reservations. Mersenne was affected by such decisions of the Church, but less so than one might suppose. The purpose of this paper is to determine how much he was affected by them, and in what way.