The following is taken from Pope Leo XIII's encyclical Libertas, on the nature of true liberty. The topic here has sparked much controversy, particularly as it relates to Vatican II, which easily appears to be contrary to what follows here. (Whether it is in fact contrary is a big question, to which I do not have a certain answer).
[L]et us examine that liberty in individuals which is so opposed to the virtue of religion, namely, the liberty of worship, as it is called. This is based on the principle that every man is free to profess as he may choose any religion or none.
20. But, assuredly, of all the duties which man has to fulfill, that, without doubt, is the chiefest and holiest which commands him to worship God with devotion and piety. This follows of necessity from the truth that we are ever in the power of God, are ever guided by His will and providence, and, having come forth from Him, must return to Him. Add to which, no true virtue can exist without religion, for moral virtue is concerned with those things which lead to God as man's supreme and ultimate good; and therefore religion, which (as St. Thomas says) "performs those actions which are directly and immediately ordained for the divine honor",(7) rules and tempers all virtues. And if it be asked which of the many conflicting religions it is necessary to adopt, reason and the natural law unhesitatingly tell us to practice that one which God enjoins, and which men can easily recognize by certain exterior notes, whereby Divine Providence has willed that it should be distinguished, because, in a matter of such moment, the most terrible loss would be the consequence of error. Wherefore, when a liberty such as We have described is offered to man, the power is given him to pervert or abandon with impunity the most sacred of duties, and to exchange the unchangeable good for evil; which, as We have said, is no liberty, but its degradation, and the abject submission of the soul to sin.
21. This kind of liberty, if considered in relation to the State, clearly implies that there is no reason why the State should offer any homage to God, or should desire any public recognition of Him; that no one form of worship is to be preferred to another, but that all stand on an equal footing, no account being taken of the religion of the people, even if they profess the Catholic faith. But, to justify this, it must needs be taken as true that the State has no duties toward God, or that such duties, if they exist, can be abandoned with impunity, both of which assertions are manifestly false. For it cannot be doubted but that, by the will of God, men are united in civil society; whether its component parts be considered; or its form, which implies authority; or the object of its existence; or the abundance of the vast services which it renders to man. God it is who has made man for society, and has placed him in the company of others like himself, so that what was wanting to his nature, and beyond his attainment if left to his own resources, he might obtain by association with others. Wherefore, civil society must acknowledge God as its Founder and Parent, and must obey and reverence His power and authority. Justice therefore forbids, and reason itself forbids, the State to be godless; or to adopt a line of action which would end in godlessness-namely, to treat the various religions (as they call them) alike, and to bestow upon them promiscuously equal rights and privileges. Since, then, the profession of one religion is necessary in the State, that religion must be professed which alone is true, and which can be recognized without difficulty, especially in Catholic States, because the marks of truth are, as it were, engrave[d] upon it. This religion, therefore, the rulers of the State must preserve and protect, if they would provide - as they should do - with prudence and usefulness for the good of the community. For public authority exists for the welfare of those whom it governs; and, although its proximate end is to lead men to the prosperity found in this life, yet, in so doing, it ought not to diminish, but rather to increase, man's capability of attaining to the supreme good in which his everlasting happiness consists: which never can be attained if religion be disregarded.