13 It is not our intention here to repeat the arguments which clearly expose the errors of Socialism and of similar doctrines. Our predecessor, Leo XIII, most wisely did so in truly memorable Encyclicals; and you, Venerable Brethren, will take the greatest care that those grave precepts are never forgotten, but that whenever circumstances call for it, they should be clearly expounded and inculcated in Catholic associations and congresses, in sermons and in the Catholic press. But more especially - and We do not hesitate to repeat it - by the help of every argument, supplied by the Gospels or by the nature of man himself, or by the consideration of the interests of the individual and of the community, let us strive to exhort all men, that in virtue of the divine law of charity they should love one another with brotherly love. Brotherly love is not calculated to get rid of the differences of conditions and therefore of classes - a result which is just as impossible as that in the living body all the members should have the same functions and dignity - but it will bring it to pass that those who occupy higher positions will in some way bring themselves down to those in a lower position, and treat them not only justly, for it is only right that they should, but kindly and in a friendly and patient spirit, and the poor on their side will rejoice in their prosperity and rely confidently on their help - even as the younger son of a family relies on the help and protection of his elder brother.
Source: Ad Beatissimi Apostolorum (Vatican.va)