The New Testament letters were occasional documents, meaning an occasion prompted their composition. Jude states the occasion in vv. 3-4 of his letter:
Beloved, although I was very eager to write to you about our common salvation, I found it necessary to write appealing to you to contend for the faith that was once for all delivered to the saints.
For certain people have crept in unnoticed who long ago were designated for this condemnation, ungodly people, who pervert the grace of our God into sensuality and deny our only Master and Lord, Jesus Christ.
Jude first gives the content of his appeal (v. 3: “to contend for the faith”) and then the reason for it (v. 4: “For certain people have crept in…”). Some of the themes in these two verses (e.g. salvation, intruders, condemnation, ungodliness, perverting grace) are worked out as the letter progresses.
The readers must wake up and realize the dire situation they’re in. Contending is called for. Intrusion has occurred. Grace is being perverted. Jesus is being denied.
Whatever Jude initially intended to write about, everything has now changed.