Leo XIII on When Christendom Existed

There was once a time when States were governed by the philosophy of the Gospel. Then it was that the power and divine virtue of Christian wisdom had diffused itself throughout the laws, institutions, and morals of the people, permeating all ranks and relations of civil society. Then, too, the religion instituted by Jesus Christ, established firmly in befitting dignity, flourished everywhere, by the favor of princes and the legitimate protection of magistrates; and Church and State were happily united in concord and friendly interchange of good offices. The State, constituted in this wise, bore fruits important beyond all expectation, whose remembrance is still, and always will be, in renown, witnessed to as they are by countless proofs which can never be blotted out or ever obscured by any craft of any enemies.

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Source: Leo XII, Encyclical Immortale Dei, Nov. 1, 1885, no. 21, http://w2.vatican.va/content/leo-xiii/en/encyclicals/documents/hf_l-xiii_enc_01111885_immortale-dei.html.