21 Nor can it be said that these atrocities are a transitory phenomenon, the usual accompaniment of all great revolutions, the isolated excesses common to every war. No, they are the natural fruit of a system which lacks all inner restraint. Some restraint is necessary for man considered either as an individual or in society. Even the barbaric peoples had this inner check in the natural law written by God in the heart of every man. And where this natural law was held in higher esteem, ancient nations rose to a grandeur that still fascinates - more than it should - certain superficial students of human history. But tear the very idea of God from the hearts of men, and they are necessarily urged by their passions to the most atrocious barbarity.
Source: Divini Redemptoris (Vatican.va)