The guiding principles in the Church's treatment of heretics are the following: Distinguishing between formal and material heretics, she applies to the former the canon, "Most firmly hold and in no way doubt that every heretic or schismatic is to have part with the Devil and his angels in the flames of eternal fire, unless before the end of his life he be incorporated with, and restored to the Catholic Church." No one is forced to enter the Church, but having once entered it through baptism, he is bound to keep the promises he freely made. To restrain and bring back her rebellious sons the Church uses both her own spiritual power and the secular power at her command. Towards material heretics her conduct is ruled by the saying of St. Augustine: "Those are by no means to be accounted heretics who do not defend their false and perverse opinions with pertinacious zeal (animositas), especially when their error is not the fruit of audacious presumption but has been communicated to them by seduced and lapsed parents, and when they are seeking the truth with cautious solicitude and ready to be corrected" (P.L., XXXIII, ep. xliii, 160). Pius IX, in a letter to the Bishops of Italy (10 Aug., 1863), restates this Catholic doctrine: "It is known to Us and to You that they who are in invincible ignorance concerning our religion but observe the natural law . . . and are ready to obey God and lead an honest and righteous life, can, with the help of Divine light and grace, attain to eternal life . . . for God . . . will not allow any one to be eternally punished who is not wilfully guilty" (Denzinger, "Enchir.", n. 1529). X.