When Jude wrote this epistle, every body of truth that was part of the revelation of God and the salvation that we have, was established and had been revealed. Once for all there would never need to be another revelation. There would never need to be a Koran. There would never need to be a Book of Mormon. There would never need to be some other sacred holy text. No, the truth of salvation through Christ, which is the only salvation, had been delivered once for all by the time Jude was writing.1
John McKnight
3 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.