Chapter 11 — Part I: The Creed — Article IX: The Holy Catholic Church; the Communion of Saints
Section 3. The second consideration is that he whose mind is strongly impressed with the truth taught in this Article, will easily escape the awful danger of heresy. For a person is not to be called a heretic as soon as he shall have offended in matters of faith; but he is a heretic who, having disregarded the authority of the Church, maintains impious opinions with pertinacity. Since, therefore, it is impossible that anyone be infected with the contagion of heresy, so long as he holds what this Article proposes to be believed, let pastors use every diligence that the faithful, having known this mystery and guarded against the wiles of Satan, may persevere in the true faith.