The book of Jude is placed at the end of the epistles, just before the book of Revelation.
It is both instruction for the early churches and foreshadows what will happen to the Church and to Christians in the last days.
Jude, the half-brother of Jesus, wrote this letter with one main purpose:
"That ye should earnestly contend for the faith which was once delivered unto the saints” (Jude 1:3).
1 John 5:16 If any man see his brother sin a sin which is not unto death, he shall ask, and he shall give him life for them that sin not unto death. There is a sin unto death: I do not say that he shall pray for it.
17 All unrighteousness is sin: and there is a sin not unto death.
John teaches that all unrighteousness is sin, but he differentiates among sins.
There is a sin unto death and a sin not unto death.
Hebrews 6:4 For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost,
5 And have tasted the good word of God, and the powers of the world to come,
6 If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.
The words "if they shall fall away" scares many believers, who, like the apostles at the Last Supper, keep asking "Is it I, Lord?"