12 When the twofold principle of cohesion of the whole body of society has been weakened, that is to say, the union of the members with one another by mutual charity and their union with their head by their dutiful recognition of authority, is it to be wondered at, Venerable Brethren, that human society should be seen to be divided as it were into two hostile armies bitterly and ceaselessly at strife? Drawn up against those who possess property, whether by inheritance or by industry, stand the proletariate and the workers, inflamed with hatred and envy, because, although they are by nature the same, they do not occupy the same position as the others. Once they have been imbued with the fallacies of the agitators, to whose behests they are most docile, who will ever make them see that it does not follow that because men are equal by their nature, they must all occupy an equal place in the community? And further, who will ever make them see that the position of each one is that which each by use of his natural gifts - unless prevented by force of circumstances - is able to make for himself? And so the poor who strive against the rich as though they had taken part of the goods of others, not merely act contrary to justice and charity, but also act irrationally, particularly as they themselves by honest industry can improve their fortunes if they choose. It is not necessary to enumerate the many consequences, not less disastrous for the individual than for the community, which follow from this class hatred. We all see and deplore the frequency of strikes, which suddenly interrupt the course of city and of national life in their most necessary functions, we see hostile gatherings and tumultous crowds, and it not unfrequently happens that weapons are used and human blood is spilled.
Source: Ad Beatissimi Apostolorum (Vatican.va)