4 And if it is true that any Christian State does something eminently disastrous and reprehensible in separating itself from the Church, how much more deplorable is it that France, of all nations in the world, would have entered on this policy; France which has been during the course of centuries the object of such great and special predilection on the part of the Apostolic See whose fortunes and glories have ever been closely bound up with the practice of Christian virtue and respect for religion. Leo XIII had truly good reason to say: "France cannot forget that Providence has united its destiny with the Holy See by ties too strong and too old that she should ever wish to break them. And it is this union that has been the source of her real greatness and her purest glories.... To disturb this traditional union would be to deprive the nation of part of her moral force and great influence in the world."
Source: Vehementer Nos (Vatican.va)