The Bible often uses the word brethren to indicate brothers. It is a fancy plural form of "brother" most often used formally when speaking in a religious tone.
The plural noun brethren is an archaic word that was common in the Middle Ages, and carries a religious context. Some religious sects use the word in their name, such as the Mennonite Brethren Church. Because of the formal, religious sense of the word, it is also used today among male members of fraternal organizations, especially those with religious ties. Thomas Paine used it to argue for democracy when he wrote, “The world is my country, all mankind are my brethren, and to do good is my religion.”