Why might Jude vs. 3 tell us that "faith" had been fully delivered to the saints?
Answer: Far more than "faith" is meant here. The statement in Jude vs. 3 applies to all that is contained in the Word of God.
While it is true that faith applies to our common salvation in Christ, there is a much more profound meaning than the superficiality of mere "faith." Jude's reference to "faith" is characterizing the totality of that which Christians believe. The word "faith" is used to encompass the entire, recognized body of teaching that emerged from Peter's initial preaching on the day of Pentecost.
And, this includes all the basic N/T requirements of faith. A substantial case may be made that faith is composed of the following commandments to which the faithful are accountable:
1. Hearing the Word of God (Romans 10:17). That would be the totality of Scripture — all of that which has been delivered. (Jude vs. 3, cf. John 6:44-45).
2. Belief in Jesus as the Son of God (Hebrews 11:6, John 8:24, 20:30-31).
3. Repentance of sin (Acts 2:38, Acts 17:30).
4. Confession that Jesus is the Son of God (Acts 8:36–37, Romans 10:9,10).
5. Baptism baptism now saves you (1 Pet. 3:21, Gal. 3:27, Acts 2:38, 8:12, 8:16, 8:36, 10:48, 16:15, 16:30, 19:3-5, 22:16, Mark 16:16, Romans 6:3, Titus 3:5).
6. Living a life of faithful obedience (1 Peter 2:9, Colossians 1:22-23).
Faith is also comprised of our obligations to worship God on the first day of the week (Sunday) and to observe the Lord's Supper (communion). All of this is accomplished as we internalize the Holy Spirit — the words of Scripture.
Coffman suggests the following:
Here again, in the New Testament usage of faith, it means, as so frequently in other New Testament passages, as Alford put it: "Faith means the faith which is believed, not the faith by which we believe."1
The use of the Greek word [hapax] carries the meaning of "once only and forever." The gospel delivered to mankind was not a piecemeal revelation, "here a little and there a little" as in the Old Testament, but the full message in its entirety and completeness as delivered through Christ to the apostles. The word (Greek: hapax) is the same as in such New Testament expressions as "appointed to man once to die," "Christ offered himself once," etc... The gospel was delivered not in part, but as a complete whole.
If faith has been delivered once and for all, the most relevant meaning is that the source of faith, the Word of God, has been fully delivered in its totality. This would then parallel Paul's Second Letter to Timothy:
2 Timothy 3:16: "All Scripture is inspired by God and profitable for teaching, for reproof, for correction, for training in righteousness" as statements that indicate the complete knowledge of God having been bestowed upon us once and forever.
If we did not have "all Scripture" as Paul maintains, we would not be so equipped. This seems to be Jude's point in the third verse of his Letter — as "perplexing a thinking pattern" as one especially useful contributor suggests.
1 Delbert R. Rose, a quotation from Alford, op. cit., p. 432.