Once and For All.

Jude, the half brother of Jesus wrote in his epistle, “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.” (Jude 3, ESV).
 
That phrase, "once for all delivered to the saints" speaks of a "faith" -- a body of known and definable doctrine. "The faith is shown to be synonymous with the apostolic preaching by the use of delivered (Gr paradidō) which is a technical term for the preaching or handing down of the gospel message by the apostles (the word is used twice in I Corinthians 15:3 for the message which Paul "delivered" after having "received" it himself)" [KJV Bible Commentary].
 
By the time Jude wrote this letter, there was a clear, authoritative body of teaching on the Christian faith that was discernible, definable, defendable and unalterable. "A further implication of this verse is that, after the writings authorized by the apostles were included in the NT canon, nothing more could ever be added to Scripture, since the content of the faith had been delivered 'once for all'" [ESV Study Bible].
 
This makes it very clear that any person or institution that seeks to add doctrine to this completed body of teaching that we call the Apostolic faith or the New Testament, is clearly false.  "Together with the Old Testament (Eph. 2:20), this apostolic witness, as found in the New Testament, is the standard for the church (2 John 9, 10)" [Reformation Study Bible]. More specifically the addition to inspired truth by any means suggesting that this addition is inspired and authoritative is false.  This discounts such claims from Seventh Day Adventists, Mormons, Roman Catholics and Charismatics who believe their insights are on a par with Scripture.