Contending . . . for the faith of the gospel immediately calls to mind the opposition. So Paul adds, without being frightened in any way by those who oppose you. The (somewhat rare) word translated frightened, used sometimes to refer to "spooking" horses, probably here carries the nuance "be intimidated" or "be thrown into consternation."
But what kind of opposition would possibly intimidate the Philippian believers? Although we cannot be certain, the best guess is related to the fact that Philippi was a Roman military colony, whose populace for very good historical reasons were devoted to the emperor (see introduction). In fact the cult of the emperor, whose "divine" titles were "lord" and "savior," apparently flourished in Philippi, so that every public event also served as an opportunity to proclaim "Caesar is lord"—in very much the same way as "The Star-Spangled Banner" or "O Canada!" is sung before public events in North America.
The problem for believers is obvious—and would easily arouse suspicions as well as hostility. For they were devoted to another Lord and Savior (Phil 3:20 is the only place this combination occurs in Paul's letters) and would find proclaiming Caesar as lord to be an impossible conflict of devotion. To top it all off, their Lord had in fact been crucified by the Roman "lord," thus branding him forever as an enemy of the state, of the insurrectionist type. Thus believers in Christ could scarcely be more out of touch with the sympathies of the local populace than in a place like Philippi. Hence Paul's concern that those who oppose them not intimidate or throw them into consternation.
This in part accounts for the constant focus on Christ in this letter. He who, following his crucifixion, currently reigns as Lord (2:9) will someday return from heaven as Lord and Savior (3:20-21), before whom every knee will bow—including that of the current "lord Caesar," Nero himself, who will join with all others to confess that the only Lord is none other than Jesus Christ (2:10-11).
Paul's mention of the opposition now prompts him to insert a parenthetical moment, describing what their standing up to the opposition will mean, as by the Holy Spirit they form a united front in contending for . . . the gospel in Philippi. Such "Spiritual" boldness on their part will serve as a sign (perhaps "omen") to them that they will be destroyed, but that you will be saved.
How so, Paul does not say, but the answer probably lies with the Philippians' embracing the eschatological certainties just given in verses 21-24, which will give to them, as to Paul, uncommon boldness in proclaiming Christ in Philippi. Such people cannot be intimidated by anyone or anything, since they belong to the future with a kind of certainty that people whose lives are basically controlled by Fate could never understand—surely an "omen," Paul says, using the language of their own worldview, that they are headed for destruction.
At the same time, of course, such resolve and unity in the face of opposition will fall out as salvation for the Philippians. Although the grammar is a bit sticky here, most likely the word saved ("salvation") carries a sense very close to that in verse 19. Such salvation/vindication will not necessarily be manifest to the opponents, but it will become clear to the believers themselves. To drive this point of assurance home, Paul adds, "And this (their destruction and your salvation/vindication) comes from none other than God himself."
As always God is both the first and the last word. Salvation is at his initiative; it comes from him. Thus it is the first word. But in this sentence, in light of the Philippians' need for reassurance, it is the last word as well. Everything is from God; the Philippians can rest assured. Which is also a necessary word in light of the theological explanation of suffering that Paul is about to offer.