5 The natural tendency of man to association has never been stronger, or more earnestly and generally followed, than in our own age. This is not at all to be reprehended, unless when so excellent a natural tendency is perverted to evil purposes, and wicked men, banding together in various forms of societies, conspire "against the Lord and against His Christ" (Ps ii., 2). It is, however, most gratifying to observe that pious associations are becoming more and more popular among Catholics also. They are frequently formed; indeed, all Catholics are so closely drawn" together and united by the bonds of charity, as members of one household, that they both may be and are truly styled brethren. But if the charity of Christ be absent, none may glory in the name and fellowship of brethren. So wrote Tertullian long ago in pungent words: "We are your brethren by right of a common mother, nature, yet are ye less than men, because unnatural brothers. How much more justly are they called and esteemed as brethren who acknowledge one and the same Father, God; who have drunk in one and the same spirit of charity; who have been borne from one and the same womb of ignorance into the one light of truth?" (Apolog. c. xxxix.)
Source: Augustissimae Virginis Mariae (Vatican.va)