Jude gives us a typical greeting in the first 2 verses, telling us who is writing and extending his good wishes in love to his recipients. Then he quickly moves to his purpose for writing:
Dear friends, although I was very eager to write to you about the salvation we share, I felt compelled to write and urge you to contend for the faith that was once for all entrusted to God’s holy people. For certain individuals whose condemnation was written about long ago have secretly slipped in among you. They are ungodly people, who pervert the grace of our God into a license for immorality and deny Jesus Christ our only Sovereign and Lord.
Although he wanted to write to them about the salvation we share in Christ, events have intervened. Notice the words he uses here, he felt “compelled” to write them to “urge” them to “contend” for the faith; words that imply a crisis of some sort. “Contend” comes from a word that would normally be used in either a military or athletic contest. Yes, there would seem to be a crisis at hand.
That faith he wants them to contend for is the faith that was “once for all” entrusted to “God’s holy people.”
This statement is reminiscent of John’s “as you have heard from the beginning” the message of faith had been given to them by others when they came to believe the gospel; it does not change. You could say that Jude isn’t going for any of the present-day notions of progressive revelation wherein the message changes or evolves over time. The gospel is the gospel, once and for all time.
There seems to have been certain people who have come within the church who have ideas about changing the message. We can clearly see two things from verse 4: These people wish to abuse the liberty we have in Christ to engage in immoral practices, and to deny Christ. At this point, they sound quite a bit like “antichrist” in 1 and 2 John, and you will recall that they were Gnostics.
There is always a tension between liberty and going too far. We have a great freedom in Christ, since the Law is gone and His laws are “written on their hearts.” Can you have one “law” and I another− scholars have debated this for centuries: What really is the difference between “liberty” and “license”? In verse 4 it would appear that one field of conflict was that of sexual practices, one that will be repeated in the verses that follow, and one that is very much under discussion in our day. Where is that line?
In recent centuries within Western culture, that line was well-defined with lists of things you mustn’t do: “Don’t smoke or chew or go with girls who do” is one of the more amusing of these. I prefer another approach: Does an action glorify God? Does it build up His Kingdom, or will it tear the Kingdom apart? Apparently, Jude felt strongly that the Kingdom was in jeopardy.