Following his opening, he begins his letter by saying, "I really wanted to write something theological to you, something about our salvation, but this matter came up, and I feel that it is more necessary to write to you about this more pressing problem."
In the strongest possible terms, he tells them to fight, to strive, to struggle for the truth as it had been given to them by the apostles, who in turn had been taught by Christ Himself. The word translated "contend earnestly," epagonizomai, is very interesting and picturesque. We have a word in English that derives from its root: "agony." Epagonizomai means literally "to agonize about," thus "to contest" or "to contend." It describes the efforts of an athlete to win his particular competition in the midst of the games, whether it was running, javelin throwing, discus, or whatever the particular sport. An athlete who is truly devoted and focused on winning gives his every ounce of strength to come out on top. Jude uses epagonizomai to demonstrate how we should be exerting ourselves in keeping the truth pure and practicing it. It suggests a person straining for all he is worth to ensure the faith's purity in both its principles and practices. This struggle is hard and painful at times—sometimes even deadly—but the truth is that important to God and to our brethren, the church.
Jude also affirms in this verse that the truth we need for salvation has already been given once for all. This is a very important distinction because it lays the foundation for what he writes later. The truth was given—and that is that! There is no continuing revelation, no evolution of truth. It was closed by the passing of the apostles, specifically the Twelve.
We could paraphrase this with, "Beware of those who say they have 'new' truth." Refinements of or gleaning deeper meanings from an "old truth" are fine, but claims of new truth should raise red flags. They should sound sirens and flash lights in our mind. The truth, the faith, has been delivered once for all. We should cling for all we are worth to the Scriptures and should not listen to those who claim to have special revelation. The Bible should always be the basis of our belief in anything spiritual. If a teaching does not square with the Bible, we should reject it; anything contrary to the faith once delivered should be thrown out as soon as possible.