Jude has been attributed to Jude (Judas), younger brother of Jesus and James from earliest times. He writes as an old man late in the first century. The purpose of the letter is to refute some false teaching that has arisen in churches under his care. Jude parallels 2 Peter and quotes from at least two extra canonical sources (The Assumption of Moses and The Book of Enoch.) This is a cyclical letter and was intended to be read in many churches. Key Learning: Right doctrine leads to right living.
The author does not identify the actual "false teaching" but again, like 2 Peter we can surmise the heart of the difficulty. People have come saying that what we do with our bodies has no consequences because the flesh is not eternal and will not go on to glory anyway. This teaching rises out of Gnosticism. The Gnostics taught a dualistic understanding of the universe. There is good and there is evil and they are relatively equal powers. This was later refined to teach that all matter (all material things) are evil and that only the Mind or Spirit is good. They taught, consequently, that Jesus, the Son of God, could never have stooped to become flesh and blood for to do so was to put on evil. Therefore, they taught, he only "appeared" to be a human. If the flesh is of no consequence than whatever I do -- indulge in whatever kind of orgy I choose -- has no consequence because only the mind and spirit matter. This teaching exists today in the teachings of Christian Science.
This is in contrast to a core Christian doctrine called the Incarnation. The Bible tells us that God created the material world and called it "good" (see Genesis 1 and 2). We know from the Gospels that God became flesh and dwelt among us -- that Jesus was God with skin on. Matter and material things are not inherently evil. I am a created being -- and uniquely created. God made the animals to be pure flesh. God made the angels to be pure spirit. God made me to be both at the same time -- flesh and spirit united in one body. What I do with my body has a direct impact on my spiritual side -- what I do with my spirit also has an impact on my flesh. They cannot be separated -- until we pass through death to eternity.