Jude tackles the same problems Peter did in his second letter: false teachers who were leading the early church astray. "Murmurers" and "complainers" who were "walking after their own lusts" (1:16) were apparently using the grace of God as a cover for their sinful lifestyles and encouraging Christian believers to do the same. True believers, Jude says, reflect God's love, show compassion, and work to pull sinners "out of the fire"(1:23).
Ye should earnestly contend for the faith which was once delivered unto the saints. (1:3)
Satan tries to sneak "secret agents" into God's church to confuse and ultimately crush true believers. It's the job of every true Christian to "earnestly contend for the faith" as passed down by Jesus' disciples and recorded in the Bible.