9 It will be your duty, venerable brothers, to impress diligently upon the minds of your people these points which I have briefly touched upon, that they not rashly follow false notions concerning dueling, nor allow themselves to be carried away by the judgments of shallow men. Take particular care that youth at the right time understand that the Church's position on dueling is in agreement with natural philosophy. Indeed, just as in other countries Catholics in the prime of youth voluntarily and faithfully refuse to endorse dishonorable associations, in like manner, we are extending to Catholic youth the opportunity to make the same agreement among themselves and pledge that at no time and under no condition will they engage in a duel.
Source: Pastoralis Officii (Vatican.va)