The “faith once for all delivered to the saints” is not a mark of early Catholicism but a statement about the universality of Christian belief. The “faith” here is the content of what Christians believe. This faith has been “delivered,” a word used to refer to the authoritative handing-down of teaching or tradition. With the addition of the word that means “once for all” any modern addition to the Christian faith is excluded, leaving one to look to the apostles and the apostolic tradition as the source of true teaching regarding the faith.
Any modern teaching or preaching must conform to the apostolic tradition (this is “tradition” in the good and positive sense) in order to be validated or authenticated. Preaching or teaching that is in line with apostolic tradition is true Christian preaching and that which disagrees is not true authentic Christian teaching.
This teaching is bound to the historical events of the Incarnation and the Resurrection and is based upon the eyewitness testimony of the apostles who bore witness to it and who proclaimed it and its implications in accordance with the Lord’s commission. Christian teaching in every age is inextricably bound to the apostolic proclamation of those events and their explanation of the ramifications of those events.
Jude’s emphasis on the moral failures of the false teachers, however, adds a new dimension to this understanding of the “faith.” It is more than mere doctrinal propositions (though is certainly not less than, or other than, that) but involves the moral implications of those doctrinal propositions. To fight for the gospel they must not only believe and proclaim the propositional truth of the faith, but be faithful and obedient to the moral demands placed upon them as a result of believing the gospel. These will stand in direct and stark contrast to the false teachers who not only twisted the doctrinal content of the “faith” but also ignored its moral implications.