Why was the Book of Jude written? Was Jude one of the disciples and if not, who was he? What was the purpose of the Book of Jude?
What was Jude?
Jude which in Hebrew is Judas was one of four half-brothers of Jesus Christ (Mark 6:3; Matt 13:55). He is not the same Judas as the Apostle Judas who was the son of James (Luke 6:16; Acts 1:13). Just like Jesus’ other family members, except for His mother Mary, they didn’t believe in Him (John 7:1-9) until after He was resurrected (Acts 1:14). Scholars believe that this was written before A.D. 70 because there is no mention of the fall of Jerusalem and so it was likely written in the late A.D. 60’s. This is the 4th smallest book of the Bible yet one of the most potent in exposing doctrinal errors and false teachers. 
Purpose of Jude
Jude wrote this book to the church because the church was going through a time of great apostasy and there was no shortage of doctrinal and moral error inside the church while there were other false churches robbing from the doctrine of the apostles but mixing it with error, having false teachings within them. While Peter talked about the coming of false teachers (2 Peter 2:-12; 3:3) Jude addresses them as already being present (Jude 1:4, 11-12, 17-18). Jude seems to have written this while in Jerusalem even though he did go on some missionary trips early on (1 Cor 9:5). When Jude wrote this book, the church was under heavy persecution from Rome and the Jewish leaders. Jude wrote this book exclusively to fight against the apostasy that was defiling the church and defectors of the faith who were building apostate churches. These are they who had departed from the faith that was originally delivered (Jude 1:3), if they were ever in the faith in the first place (1 John 2:19). He tells the church to “contend for the faith once delivered” because the faith was under attack. Today, we have this in common with no shortage of apostate churches.
Jude on Perverting the Gospel
Jude 1:3-4 “I found it necessary to write appealing to you to contend for the faith that was once for all delivered to the saints. For certain people have crept in unnoticed who long ago were designated for this condemnation, ungodly people, who pervert the grace of our God into sensuality and deny our only Master and Lord, Jesus Christ.”
It is still necessary to contend for the faith since many cults have arisen and take a buffet-style approach to doctrine. They take a little of this and some of that but leave out the more crucial elements making it not the whole counsel of God but a perverted one that is not a gospel at all but teaching from the Devil. These “people have crept in unnoticed” by most people but when their doctrines are held up against the Word of God, they can be seen as false. They “pervert the grace of God into sensuality and deny” Jesus’ teachings.
Jude on the Coming Judgment
Jude 1:6-7 “And the angels who did not stay within their own position of authority, but left their proper dwelling, he has kept in eternal chains under gloomy darkness until the judgment of the great day— just as Sodom and Gomorrah and the surrounding cities, which likewise indulged in sexual immorality and pursued unnatural desire, serve as an example by undergoing a punishment of eternal fire.”
These “angels who did not stay in their own position of authority” are the fallen angels (demons)  that may have been so wicked that God had to take them out of the world and place them “in eternal chains…until the judgment of that great day” (Rev 20:10, 15) when they will be cast into the lake of fire.
Jude on the Prosperity Gospel
Jude 1:10-13 “But these people blaspheme all that they do not understand, and they are destroyed by all that they, like unreasoning animals, understand instinctively. Woe to them! For they walked in the way of Cain and abandoned themselves for the sake of gain to Balaam’s error and perished in Korah’srebellion. These are hidden reefs at your love feasts, as they feast with you without fear, shepherds feeding themselves; waterless clouds, swept along by winds; fruitless trees in late autumn, twice dead,uprooted; wild waves of the sea, casting up the foam of their own shame; wandering stars, for whom the gloom of utter darkness has been reserved forever.”
These false teachers walk in the way of Cain who openly rebelled against God by not giving God a proper offering and then murdering his brother Able (Gen 4:1-15; 1 John 3:12). It is for “the sake of gain to Balaam’s error” meaning that like Balaam enticed Israel into idolatry and sexual immorality for money (Num 22-25); today these prosperity preachers seek to line their own pockets while promising their followers prosperity if they donate to their cause which causes others to make money their god. Their end will be like that of Korah where his rebellion caused the earth to swallow him and his followers (Num 16:1-12). They are like “hidden reefs” which cannot be seen but destroy great ships that collide with these unseen “reefs” and like “waterless clouds” they look good but are really worthless. They are “swept along by winds (culturally relevant) fruitless trees (good for nothing but to be burned)” and “twice dead” meaning that they were never really born again (John 3:3) but only born of a woman which means if a person is born again, they will die only once but if a person is not born again, they will die twice; once physically and finally eternally (Rev 20:11-15). They have reservations for the eternal outer darkness where there will be weeping and gnashing of teeth that Jesus so often spoke about (Luke 13:28).
Jude on Discontent, Greed, and Pride
Jude 1:14-16 “Behold, the Lord comes with ten thousands of his holy ones, to execute judgment on all and to convict all the ungodly of all their deeds of ungodliness that they have committed in such an ungodly way, and of all the harsh things that ungodly sinners have spoken against him. These aregrumblers, malcontents, following their own sinful desires; they are loud-mouthed boasters, showing favoritism to gain advantage.”
The Lord is coming to judge these false teachers and their followers. The fact that Jude quotes Enoch’s prophecy, which is an extra-biblical book, does not mean what Jude quotes isn’t Holy Spirit inspired. Today there is no shortage of those who are “grumblers (against church leaders), malcontents (those who are never content), following their own sinful desires (only fruit is fruit of the flesh); they are loud-mouthed boasters (braggarts boasting of themselves, never giving glory to God), showing favoritism (to the rich) to gain (financial) advantage.” This sounds so much like the prosperity gospel.
Jude on Doubters
Jude 1:17-19 “But you must remember, beloved, the predictions of the apostles of our Lord Jesus Christ. They said to you, “In the last time there will be scoffers, following their own ungodly passions.” It is these who cause divisions, worldly people, devoid of the Spirit.”
Here Jude is acknowledging the writings of the other apostles who warned before about these “scoffers” (like, “God does not exist”) and who “cause divisions (in churches like the Corinthian church; 1 Cor 3:1-9), worldly people (carnal, claiming to be Christians but are not, e.g. Matt 7:21-23), devoid of the [Holy] Spirit.”
Jude on New Believers and Evangelizing
Jude 1:22-23 “And have mercy on those who doubt; save others by snatching them out of the fire; to others show mercy with fear, hating even the garment stained by the flesh.”
Jude shows that we must show mercy for those believers who doubt their own salvation  or doubt at times whether God actually exists. Haven’t you ever doubted your own salvation or the existence of God at times? I have. This is particularly important for new believers. In witnessing to the lost or to “save others” we literally “snatch them out of [hell] fire” telling them about the abiding wrath of God on sinners enabling them to see their sin (to humble them) and their need for the Savior while for others showing them mercy if they are already humbled.
Jude concludes by telling Christians that God “is able to keep you from stumbling and to present you blameless before the presence of his glory with great joy, to the only God, our Savior, through Jesus Christ our Lord, be glory, majesty, dominion, and authority, before all time and now and forever. Amen” (Jude 1:24-25). Either He will present you to God in glory and you will have great joy or you will be “undergoing a punishment of eternal fire” (Jude 1:7). The choice is up to you.
Read this other commentary, also by Pastor Jack: Romans Commentary, Summary and Key Verses 
Resource – Scripture quotations are from The Holy Bible, English Standard Version® (ESV®), copyright © 2001 by Crossway, a publishing ministry of Good News Publishers. Used by permission. All rights reserved.