Denying the Lord: Rejecting Messiah

Jude's opponents are not outsiders. They are operating inside the wider church, and what he accuses them of is denying our only Master and Lord, Jesus Christ.
The sources read that denial as christological rather than merely moral. It is a rejection of his deity.
The phrase has an ancestor in the Enochic literature, where judgment falls on those who have denied the Lord of Spirits. Jude's readers knew that world, which is part of why his letter sounds the way it does.
A note on citation. The denial formula appears at 1 Enoch 45:2. If you have seen it attributed to 48:10, that reference does not carry the phrase.
Scripture links:
- Jude 4 - certain people have crept in, who deny our only Master and Lord.
The danger Jude describes was never outside the door. It had already come in and sat down.