The subject matter of Jude’s letter was originally going to be “the common salvation,” presumably meaning he was going to explore that subject matter. This current letter touches on that subject, but rather than exploring “the common salvation” in detail, he issues a call to action urging his readers to defend the faith (which is grounded in that common salvation).
The phrase “common salvation” reminds the reader of the corporate nature of the Christian faith. To be a Christian, one must agree with what Christians have believed at all times and in every place. “Salvation” is the deliverance of God’s people from sin and judgment. It is both present, in that God’s people enjoy the benefits of salvation and reconciliation with god in this life, and eschatological, pointing toward the ultimate salvation (v. 21). These are significant in this context because, as the rest of epistle makes clear, the false teachers are perverting the nature of sin, judgment, and salvation.
Jude’s epistle focuses quite a bit on the eschatological (that is ultimate and end-of-world) judgment and accountability. He looks backward to provide an example of what is to come when the Lord returns “with ten thousand of his saints to execute judgment on all” (v. 14-15). So when Jude speaks of our “salvation,” he is speaking of our deliverance from the wrath of God. He wants them to be pure so they will enjoy “the mercy of our Lord Jesus Christ unto eternal life” (v. 21) which we will enjoy if we are found “faultless before the presence of his glory” (v. 24).
That this is the common faith makes clear that it is held by those everywhere and in all times who have been delivered from the wrath of God. This leaves no room for new interpretations or understandings but only for proclamation of what God himself has revealed to deliver us from wrath.