Jude is one of eleven New Testament books that do not have any direct quotes from the Old Testament.
[Why Was New Testament Written in Greek?]
According to a Wikipedia article on the epistle of Jude, the book apparently did not have much influence in later Christianity. It rarely appeared in sermons preached to laypeople or in treatises by Christian theologians.
[Why Did Jesus' Disciples Leave Him?]
Jude, one of the half-brothers of Jesus Christ, is often confused with Judas Thaddaeus (Lebbeus), one of Jesus' twelve apostles (Matthew 10:3, Luke 6:16). Biblical commentary references to Saint Jude are usually discussing this apostle and not the Lord's half-brother.
There are twenty-two books total that compose the New Testament sections of Paul's writings (14), the General Epistles (7) and the book of Revelation (1). Jude is one of only four of these books, the others being Hebrews, 2Peter and 1John, that does not state it was written for a particular recipient.
Similarities
The book of Jude is remarkably similar to certain sections of the book 2Peter. For example, consider the following two quotes.
But there were false prophets also among the people, even as there shall be false teachers among you, who privily shall bring in damnable heresies, even denying the Lord that bought them, and bring upon themselves swift destruction (2Peter 2:1, KJV).
For there are certain men crept in unawares, who were before of old ordained to this condemnation, ungodly men, turning the grace of our God into lasciviousness, and denying the only Lord God, and our Lord Jesus Christ (Jude 1:4, KJV).
[Life of Apostle Peter Timeline]
Verses 5 and 12 of 2Peter 1 are correspondingly similar to Jude 3 and 5. Verses 4, 6, 10 - 11, 12, 13 - 17 in 2Peter 2 are similar to Jude 6, 7, 8 - 9, 10, 11 - 13 of Jude. Lastly, 2Peter 3 verses 2 - 3, 14 and 18 are similar to Jude 17 - 18, 24 and 25.
Ungodly
The word "ungodly" appears an amazing six times in Jude's short epistle (verses 4, 15 and 18). It occurs four of these six times in a single verse!
To execute judgment upon all, and to convince all that are ungodly among them of all their ungodly deeds which they have ungodly committed, and of all their hard speeches which ungodly sinners have spoken against him (Jude 1:15, KJV).
[Why Was the New Testament Written?]
Names and Places
The short book of Jude refers to several individuals and places directly by name.
Jude refers to his brother James (verse 1), Michael the archangel and Moses (verse 9), as well as Cain, Balaam and Korah (verse 11). He also mentions Enoch and Adam (verse 14). Locations he writes about include the land of Egypt (verse 5) along with Sodom and Gomorrah (verse 7).
Michael the archangel's mention in Jude is one of only five Biblical references to this powerful angelic spirit (see Daniel 10:13, 21, 12:1 and Revelation 12:7). He is also one of only three angelic beings named in Scripture. The other two are Gabriel (Daniel 8:15 - 16) and Lucifer (Satan's name before he turned evil, Isaiah 14:12).
[What Is the Devil's Only Promise?]
Attributes of Apostasy
Jude describes several of the attributes of those who pretend to be converted Christians but are really false believers and teachers!
These pretend Christians are ungodly, perverters of God's grace and ultimately deniers of Jesus Christ (verse 4). They imagine and create foolish and false doctrines, defile themselves with perverse behavior, and reject those given either civil or spiritual authority (verse 8).
Whatever things they do not understand they curse but are willing to pursue the desires of their deceived hearts even if it corrupts them (verse 10).
[Why Did Paul Curse the High Priest?]
They trust their own views above God's, are willing to lead others into sin for gain, and unrighteously challenge the authority God gives to those he uses (verse 11).
False teachers and brethren are also unrighteous critics and fault-finders, act arrogantly, and are willing to flatter others in order to take advantage of them (verse 16).
Lastly, such deceived people found within the church seek to divide people, are worldly minded and, of course, do not have God's spirit (verse 19).
Metaphors of Apostasy
A metaphor is a figure of speech that uses a word or phrase to reference something other than its literal meaning. It is used to draw attention to similarities between the metaphor utilized and the object or idea being referenced. Jude uses several interesting metaphors in his discussion regarding those pretending to be Christians.
These are spots (Greek spilas meaning "rocks") in your feasts of charity, when they feast with you, feeding themselves without fear: clouds they are without water, carried about of winds; trees whose fruit withereth, without fruit, twice dead, plucked up by the roots;
Raging waves of the sea, foaming out their own shame; wandering stars, to whom is reserved the blackness of darkness for ever (Jude 1:12 - 13, KJV).
[Where Is the True Church of God?]
Those who are false brethren are like rocks in that they are an unseen danger to true believers. False teachers are like unstable and restless clouds in that they do not produce a blessing for others and end up being useless.
False Christians, according to Jude, are also likened to trees which do not produce the fruit expected of them when it is needed the most. They are like raging waves, unstable in all they do, and like wandering stars, in that they are restless and possess no internal peace.
Traditions
The 1911 Encyclopedia Britannica, in its article on Jude, states he had two grandsons named Zoker and James. Living in the time of Domitian's reign as Emperor (ruled 81 to 96 A.D.), it is claimed that their testimony before Roman authorities brought an end to the persecution of believers in Palestine.
A Wikipedia article on Jude states that an early Christian tradition asserts his grandsons were interrogated by Domitian. They, according to what was passed down, defended themselves as not being rebels but only poor laborers struggling to make a living from a single patch of land.
Finally, a 1913 Catholic Encyclopedia article on Jude's epistle states the following tradition about him and his grandsons.
"From a fact of Hegesippus told by Eusebius (Hist. eccl., III, xix, xx, xxii) we learn that Jude was "said to have been the brother of the Lord according to the flesh", and that two of his grandsons lived till the reign of Trajan." (Article of Jude's epistle).