39 All these benefits of Leo's Encyclical, Venerable Brethren and Beloved Children, which We have outlined rather than fully described, are so numerous and of such import as to show plainly that this immortal document does not exhibit a merely fanciful, even if beautiful, ideal of human society. Rather did our Predecessor draw from the Gospel and, therefore, from an ever-living and life-giving fountain, teachings capable of greatly mitigating, if not immediately terminating that deadly internal struggle which is rending the family of mankind. The rich fruits which the Church of Christ and the whole human race have, by God's favor, reaped therefrom unto salvation prove that some of this good seed, so lavishly sown forty years ago, fell on good ground. On the basis of the long period of experience, it cannot be rash to say that Leo's Encyclical has proved itself the Magna Charta upon which all Christian activity in the social field ought to be based, as on a foundation. And those who would seem to hold in little esteem this Papal Encyclical and its commemoration either blaspheme what they know not, or understand nothing of what they are only superficially acquainted with, or if they do understand convict themselves formally of injustice and ingratitude.
Source: Quadragesimo Anno (Vatican.va)