In the Constitution on the Church in the World Today (1962):

“The freedom of the Church is the fundamental principle in what concerns the relations between the Church and governments and the whole civil order” (n. 13)

“The Church believes that she, through each of her members and through the entirety of her community, can contribute greatly toward making the family of men and its history more human” (n. 40).

But there is a new development: “In addition, the Church gladly sets a high value on the contributions which other Christian churches and ecclesial communities have made and are making, in a united effort, toward the fulfilment of the same task” (loc. cit.).

Leo XIII never said that. Nor did he rise to the humility of the further statement: “At the same time, [the Church] holds firmly that she can be assisted to a great extent and in a variety of ways by the world itself, by individual men and by human society, through their endowments and efforts, in preparing the way for the gospel” (loc. cit.).

The mission of the Church, “is not of the political, economic, or social order; the purpose which Christ set for it is of the religious order” (n. 42).

In consequence, the Church “is not bound to any particular form of human culture, or to any political, economic, or social system” (loc. cit.).

In further consequence, her ardent wish is “that, standing in the service of the good of all, she may be able to develop freely under any form of government which recognises the fundamental rights of the person and of the family, and also recognises the exigencies of the common good” (loc. cit.).

At the same time Gaudeum et Spes condemns dictatorship: “Disapproval is voiced of those forms of government, to be found in some countries, which fetter civil and religious freedom . . . ” (n. 73). And again: “It is inhuman that political authority should assume totalitarian or dictatorial forms which do injury to the rights of the person or of associations” (n. 75)

At least the structure of the mystery can be described in these terms: “In pursuit of her salvific purpose, the Church communicates the divine life to men— not only that; a reflection of her light somehow streams forth over the whole world, and its effect is chiefly shown in that it heals and elevates the dignity of the human person, strengthens the bonds of human society, and invests the daily activity of man with a deeper meaning and import” (n. 40).

The terms are Augustinian , but with a difference. The theme of human dignity has now become central in a new way: “By no human law can the personal dignity and freedom of man be so adequately safeguarded as by the gospel of Christ committed to the Church” (n. 41). Or again: “In virtue of the gospel committed to her, the Church proclaims the rights of man; she also acknowledges and holds in high regard the dynamism of today, whereby these rights are everywhere promoted” (loc. cit.).

Yet Gaudeum et Spes specifically confines church activity to the spiritual order: “The power which the Church is able to impart to human society today consists in faith and love made operative in life. It does not consist in any sort of external control exercised by merely human means” (n. 42)

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Comment: the  emergence  of  a  realm  of  the  secular  is  a  modern  construct.  It  is  the
product  of  many  forces,  social,  economic,  political,  cultural  and  theological.  Increased levels  of  education,  democratisation,  and  economic  freedoms  all  contributed,  as  did  the cultural  emergence  of  what  we  call  secularism,  a  movement  of  thinkers  and  their  ideas
which sought not just to delimit the role of religion in society, but if possible to eliminate it
altogether. Since the breakdown of Christendom, the Reformation, the wars of religion and
the  Enlightenment,  there  has  been  no  shortage  of  thinkers  who  have  been  ready  to
proclaim the end of religion and the beginning of a new and more rational age which looks
down upon those who hold religions belief as at best an example of intellectual frailty, and
at worst a matter of bad faith.

Further Reading Ratzinger on Church and State