We come to the purpose of Jude’s writing: the gospel must not be lost. The gospel truths about Jesus Christ must be contended for. There is no common salvation, there is no salvation even if the faith should be lost or changed.
When Jude is writing about the faith, he is talking about the truths that have been communicated by the apostles. The apostles were those first followers of Jesus who lived with him during the time of his ministry on earth, and they were also specifically equipped and authorised by Jesus to teach his words to others. Indeed they were promised in John 16 that God the Holy Spirit would help them to remember and understand everything that Jesus taught. He would ensure that God’s truth was clearly preserved and proclaimed. And what we have in the New Testament today is the doctrines that were taught by them, the foundation upon which the church of Jesus Christ was built (Ephesians 2:20). That is the faith to which Jude refers. Not a dry orthodoxy or abstract knowledge but living water—truth about God that leads to salvation for those who believe.
Because this truth is so important and because there is no other way to be reconciled to God (Colossians 1:19–20), Jude is adamant that this truth must not be changed. It is not up for negotiation or editing. That which was delivered must be kept pure, it must be protected and preserved. If you discovered a cure for cancer, you would be a fool to mess with and change the formula, likewise the body of teaching that leads to eternal life.
Also note that Jude mentions that this faith, this body of teaching handed down by the apostles, was delivered once and for all. With that phrase he is conveying the simple idea that something has happened in the past and is now finished. An event has taken place and that event has been completed. Normally we find the same kind of language used in reference to Jesus’ death on the cross (e.g. 1 Peter 3:18). Jesus bore the sins for all God’s people on the cross once and for all, so that when he said, it is finished
he meant that all sin had been dealt with. There is no more penance to pay, no purgatory, no need for Christ to be re-sacrificed in the mass. In fact any attempt to add to Jesus’ work would be to distrust and devalue it. Also the body of teaching given by the apostles is finished. We have the finished work of Christ on the cross, and in the Bible we have the finished Word or revelation of Christ. Any attempt to add to or change that faith is to distrust and devalue Christ himself.
Practically that means we do not need any new dreams or visions to know what God is saying to us today. We do not need prophets to come with new revelations. The canon of Scripture is closed. One cannot affirm the words of Jude here in Jude 1:3, where he says that the faith was delivered once and for all to the saints, and in the same breath claim that there is new revelations. God’s Word—the Old and the New Testament—contain everything we need to know about who God is and what he wants from us (see Belgic Confession, Articles 2-7). The Bible gives us the privilege of becoming wise for salvation through faith in Jesus Christ (2 Timothy 3:15).
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.