"To spread the faith" means to share or promote a particular religious belief or ideology with others. It involves sharing the teachings, values, and practices of a religion in order to gain more followers or believers. Full definition
Peter Faber, SJ, one of the great figures of the Catholic Reformation; and by doing so, gave us a key to understanding his own approach to the new evangelization, and a model for spreading the faith in modern times. (firstthings.com)
The University of Halle, which had been a center of Pietism and of missionary impulse in Lutheranism, was captured by rationalism, at the cost of some of the limited Protestant enterprise for spreading the faith. (religion-online.org)
It can involve one state with an established religion against another state with a different religion or a different sect within the same religion, or a religiously motivated group attempting to spread its faith by violence, or to suppress another group because of its religious beliefs or practices. (religion.blogs.cnn.com)