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.
--Pope Leo XIII, Libertas Praestantissimum, 1888