Bon j'ai cherché le document en question. Il s'agit d'une lettre de Léon XIII aux évêques espagnols, en date du 8 décembre 1882, intitulée Cum multa sint, et qui reproche aux catholiques espagnols leur implication dans les partis politiques au nom de la religion. C'est-à-dire exactement le contraire de ce que vous voudriez que je croie en lisant cette citation hors-contexte du pape Léon XIII.

Voici le texte intégral, que je n'ai pas trouvé en français, mais en anglais du passage consacré au lieu politique/religion.

Si je résume le 6 : le pape parle de deux extrêmes : ceux qui prônent une séparation radicale de politique et religion (ce qu'on appelle le laïcisme), et qui sont condamnables parce que leur position est pernicieuse. Dans le 7 il évoque le deuxième extrême : ceux qui mélangent tout dans les partis politiques et qui sont aussi condamnables. Lire les 8 et 9, je n'ai pas le courage d'en faire un résumé.

Dans tous les cas, je ne vois pas l'intérêt de mettre en porte-à-faux deux papes qui ont vécu à deux époques vraiment différentes, et écrit des documents dans des circonstances vraiment différentes à un public particulier. Tout le monde connaît le souci qu'avait Léon XIII de combattre le laïcisme forcené de la deuxième partie du XIXème siècle, donc certains de ses écrits sont virulents et partie intégrante d'un contexte donné.

The Relations Between Religion and Politics
6. Here, however, it will be fitting to recall the mutual relations of the spiritual and of the temporal order, for many minds on this matter fall into a two-fold error. There are some, for instance, who are not satisfied with distinguishing between politics and religion but separate and completely isolate the one from the other; they wish them to have nothing in common, and imagine that the one should exercise no influence over the other. Such men, in truth, differ but little from those who desire the exclusion of God, the Creator and Sovereign of all things, from the constitution and administration of the State; and the error they profess is the more pernicious that they thereby rashly debar the State from its most abundant source of prosperity. The moment religion is removed, those principles are of necessity shaken on which the public welfare most of all rests, and which drive their greatest force from religion, among the first of which are government with justice and moderation, obedience from a sense of duty, the submission of the passions to the yoke of virtue, to render to each his due, to leave untouched that which is another's.

7. But, though this opinion is to be avoided, the contrary error must likewise be shunned of those who identify religion with some one political party and confound these together to such a degree as to look on all of another party as undeserving any longer of the name of Catholic. This is an intrusion of political factions into the August realm of the Church; it is an attempt to break the union of brothers, and to open the gate and give access to a multitude of grievous troubles.

8. The spiritual and temporal orders being, therefore, distinct in their origin and in their nature, should be conceived and judged of as such. For matters of the temporary order - however lawful, however important they be - do not extend, when considered in themselves, beyond the limits of that life which we live on this our earth. But religion, born of God, and referring all things to God, takes a higher flight and touches heaven. For her will, her wish, is to penetrate the soul, man's best part, with the knowledge and the love of God and to lead in safety the whole human race to that City of the Future which we seek for.

9. It is, then, right to look on religion, and whatever is connected by any particular bond with it, as belonging to a higher order. Hence, in the vicissitudes of human affairs, and even in the very revolutions in States, religion, which is the supreme good, should remain intact; for it embraces all times and all places. Men of opposite parties, though differing in all else, should be agreed unanimously in this: that in the State the Catholic religion should be preserved in all its integrity. To this noble and indispensable aim, all who love the Catholic religion ought, as if bound by a compact, to direct all their efforts; they should be somewhat silent about their various political opinions, which they are, however, at perfect liberty to ventilate in their proper place: for the Church is far from condemning such matters, when they are not opposed to religion or justice; apart and removed from all the turmoil of strife, she carries on her work of fostering the common weal, and of cherishing all men with the love of a mother, those particularly whose faith and piety are greatest.