From Saint John Eudes’ book, Baptism: Man’s Contract with God:
That you may have a true faith in those things which God has revealed, it is necessary that you should believe in the Catholic Church, in which alone you can learn with certainty what God has revealed. For this reason, after you have been asked if you believe in God, you are also asked if you believe in the Catholic Church.
Certainly those who do not believe in the Catholic Church cannot have divine faith in the mysteries which they believe, but only natural and human faith; a faith of their own fancy, founded on the light of their own judgment, subject to error, and not on the promises of Jesus Christ. The Catholic Church alone possesses these promises, and on her testimony alone rests the foundation of Christian faith. As she possesses the divine promises for all days, even to the end of ages, there can be no reason to doubt whatever she proposes to our belief.
Thank God for having given you the precious gift of faith, and having made you a child of the holy Catholic Church, which is the faithful repository of the truths of salvation, and which all Christians are obliged to acknowledge as the true Church. In saying, “I believe in the holy Catholic Church’ you united yourself inseparably to this holy mother; you believe, without hesitation, all that she proposes, as proposed to you by Jesus Christ himself, who is ever with her in her instructions. Reject, then, with horror, everything at variance with her teachings, and regard it as an error calculated to endanger your faith.
However ignorant you may be, you have the true faith if you believe, without exception, all the holy Catholic Church believes and teaches; on the other hand, however learned you may be, you lose the gift and the virtue of faith if you reject any doctrine which she teaches; for her faith is your rule. “As there is but one faith,” says St. Paul, “to wish to divide it, is to destroy it.” Heretics not only differ from the Church in faith, but they also differ amongst themselves, a proof that they have not the true faith, which is one. The holy Catholic Church never has suffered, and never will suffer, a difference of faith in regard to any article. Her faith is the same in all times, in all places, and in all her true children. Thus her faith is one and the only true faith. You should be most desirous to preserve the faith in all its purity, since without it, it is impossible to do anything which merits Heaven. “Without faith it is impossible to please God.” Those who do not possess it may practice all the moral virtues, justice, sobriety, chastity, alms-deeds, prayers, mortification; and not only is this the case with heretics, but it is a truth which should be borne in mind, that these good actions, unless they have faith for their principle, will never merit Heaven for them. The law of Moses, all holy as it was, could save only those who observed it through faith.
When, therefore, you observe that those who believe not in the Church, practice some good works, offer many prayers, and lead an austere life, do not believe that they are on this account in the way of salvation, unless they have true faith; you commit an enormous sin if you believe that they can be saved outside of the Church; that they can have faith without believing in her, or that they can be saved without faith.