Jude: son of Joseph, brother of James, and half-brother of Jesus.  Jude, which is rendered "Judah" in Hebrew and "Judas" in Greek.  No date or location is suggested in this letter.  It is speculated that it was perhaps written about A.D. 65.  It is one of the few books in the New Testament addressed to all believers everywhere and at any time, as most of the other books are addressed specifically to a certain audience in that day.  Some think Jude was writing to the same audience for whom 2 Peter was intended, because of the heresy addressed by both letters.  But there is no evidence for this.

It seems that the "faith" was being attacked and jude wrote to address the attacks, and the attackers, themselves.  These men had crept in and while they professed to hold the Christian doctrine, were really undermining its faith and spreading corruption throughout the church.  The purpose, therefore, of the epistle was to put the believers on guard against the corrupt teachings of these men, and to encourage them to stand up for the principles of Christian truth.  Jude contrasts their spirit with the archangel Michael and declares that Enoch referred or prophesied of these types.  

The responsibility of the believers was to study the Word, preach the Word, and fight for its preservation.