The liberty of thinking and publishing whatsoever each one likes without hindrance is not in itself an advantage over which society can wisely rejoice. On the contrary, it is the fountainhead and origin of many evils.
Liberty is a power perfecting man, and hence should have truth and goodness as its object. But, the character of goodness and truth cannot be changed at opinion. These remain ever one and the same, and are no less unchangeable than nature herself. If the mind assents to false opinions, and the will chooses and follows after what is wrong, neither can attain its native fullness, but must fall from their native dignity into an abyss of corruption.
What ever therefore is opposed to virtue and truth may not rightly be brought temptingly before the eye of man, much less sanctioned by the favor and protection of the law. A well-spent life is the only passport to heaven, whither all are bound, and on this account the state is acting against the laws and dictates of nature whenever it permits the license of opinion and of action to lead minds astray from truth and souls away from the practice of virtue. . .
The Church of Christ is the true and sole teacher of virtue and guardian of morals. It is She who preserves in their purity the principles from which duties flow, and by setting forth most urgent reasons for virtuous life, bids us not only to turn from wicked deeds, but even to curb all movements of the mind that are opposed to reason, even though they be not carried out in action.
Pope Leo XIII
From the Encyclical Immortale Dei, November 1, 1885