ἥτις ἐστὶν αὐτοῖς ἔνδειξις ἀπωλείας, ὑμῶν δὲ σωτηρίας
For them this is a sign of destruction, but for you salvation
The majority of the recent commentators and recent English translations take it that Paul is claiming here that the Philippians’ steadfast faith in the face of opposition is a concrete manifestation to their opponents of the opponents’ destruction.
For although your loyalty to the faith is proof to them that you will perish, it is in fact proof to you that you will be saved – saved by God.
In 1:28 Paul is displaying two competing conceptions of the result of the Philippians’ adhering to their faith in the ways Paul admonishes. To the opponents, it is wilful flaunting of Roman authority and anticipates the Christians’ imminent destruction. In reality, it marks the salvation of the Christians. On this account, debates about whether the destruction/salvation pairing here refers to the temporal or eternal realm simply miss the point. The opponents view the Philippians’ physical destruction as testimony to their eternal perdition. For Paul and the Philippians, their steadfastness demonstrates their salvation, whether they live or die. It is simply the way they magnify Christ in their bodies (cf. 1:20).