From Albert G. Mackey’s Encyclopedia of Freemasonry, this installment of Symbols & Symbolism presents his exploration of what the term Brother and Brotherhood means.
The term which Freemasons apply to each other. Freemasons are Brethren, not only by common participation of the human nature, but as professing the same faith; as being jointly engaged in the same labors, and as being united by a mutual covenant or tie, whence they are also emphatically called “Brethren of the Mystic Tie.”
When our Savior designated his disciples as his brethren, he implied that there was a close bond of union existing between them, which idea was subsequently carried out by St . Peter in his direction to “love the brotherhood.” Hence the early Christians designated themselves as a brotherhood, a relationship unknown to the Gentile religions; and the ecclesiastical and other confraternities of the Middle Ages assumed the same title to designate any association of men engaged in the same common object, governed by the same rules, and united by an identical interest.
The association or Fraternity of Freemasons is, in this sense, called a brotherhood.