On the socio-political Magisterium of Leo XIII: in summary

http://www.vatican.va/holy_father/leo_xiii/encyclicals/documents/hf_l-xiii_enc_01111885_immortale-dei_en.html
http://www.vatican.va/holy_father/leo_xiii/encyclicals/documents/hf_l-xiii_enc_20061888_libertas_en.html
http://www.vatican.va/holy_father/leo_xiii/encyclicals/documents/hf_l-xiii_enc_06011895_longinqua_en.html

1.0 The State can recognise that God exists, that Christ is God and that the Catholic Church is His Church (Immortale Dei §6, Libertas §21)

1.1 Recognising that the State derives its blessings, its authority and its very existence from Christ, the State must, in justice, do Him homage (Immortale Dei §6, Libertas §21)

2.0 The State must not only not hinder the Church in her mission, but also positively assist her (Immortale Dei §6, Libertas §21)

2.1 This is because the highest good of the State’s subjects is their salvation (Immortale Dei §6, Libertas §21) and also because of the temporal blessings that follow from promoting the Church’s mission (Immortale Dei §1, 19)

2.2 There is a differentiation of the powers of the Church and of the State (Immortale Dei §10, 13, 14) (though the State’s purpose, the common good, is indirectly subordinate to the Church’s purpose, the salvation of souls—Immortale Dei §6, Libertas §21) and the Church has exclusive competence in sacred matters (Immortale Dei §11).

2.3 But ideally there should not be a separation of powers (Immortale Dei §14, 21, 22, Libertas §18, 38, Longinqua Oceani §6).

2.4 The union of Church and State is like the union between body and soul (Immortale Dei §14, Libertas §18, 38)

2.5 The Church should be separated from the State neither collectively nor in her members (Libertas §39) and the State cannot content itself only with abiding by natural truths while ignoring readily-accessible supernatural truths (Libertas §38).

2.6 The union of Church and State is ideally one in which there is “between these two powers a certain orderly connection, which may be compared to the union of the soul and body in man (Immortale Dei §14)”, that Church and State should be “happily united in concord and friendly interchange of good offices (Immortale Dei §21)”, that between the two there should be “complete harmony” (Immortale Dei §35).

2.7 Successful Church-State relations during the Middle Ages exemplify this ideal (Immortale Dei §21, 22)

2.8 Union of Church and State is desirable for the individual good of the State’s subjects (Immortale Dei §6, Libertas §21), for the common good (Immortale Dei §6, Libertas §21, though with limitations to be described in §4.0 of this post), and because of the frequent overlap between civil and sacred matters (Immortale Dei §14, §35, Libertas §18)

3 Leo reiterated the teaching of Bl. Pius IX in condemning the principles of democracy and liberalism (Immortale Dei §24, 25, 26 and 32), particularly the false liberties of worship (Libertas §20, 21), speech (Libertas §23) and conscience (Libertas §30).

4.0 Leo teaches that ideally whatever “is opposed to virtue and truth may not rightly be brought temptingly before the eye of man, much less sanctioned by the favor and protection of the law” (Libertas §32) but that, although the State is free to repress error simply because it is erroneous, the State’s higher purpose is not repression of error, but the common good, so it may permit the activity of false religions if it expects by this permission to procure a greater benefit to the common good or avert a greater damage to the common good (Immortale Dei §36, Libertas §33).

4.1 Nonetheless, “to judge aright, we must acknowledge that, the more a State is driven to tolerate evil, the further is it from perfection; and that the tolerance of evil which is dictated by political prudence should be strictly confined to the limits which its justifying cause, the public welfare, requires.” (Libertas §34, cf. Longinqua Oceani §6)

5 And the summary of the summary: the State can and must confess Christ, it must unite itself to and co-operate with His Church, and it can repress false religions, though the norm of repressing error is, for the State, subordinate to the norm of building up the common good.

Reginaldvs Cantvar
Feast of St. Eusebius, Bishop, Martyr, 2008 A.D.