Withstand him steadfast in your faith, knowing that your brothers who are in the world are undergoing the same sufferings.
Who are in the world ...  There is a poignant suggestion in this that being "in the world" was one and the same thing as being under Nero and his persecution.  Someone has said that in the times of the Caesars, the world itself was but a dreary prison for those who were proscribed by the emperor.