and not frightened in anything by your opponents. This is a clear sign to them of their destruction, but of your salvation, and that from God. (Philippians 1:28, ESV)The beginning of this verse is the end of the previous sentence. Paul begins telling them that living a life worthy of the Gospel of Jesus Christ is in unity with other believers. This unity is not compromised in the face of opposition. The word for frightened means to be startled. Don't be taken off guard by the opposition.
Unified, un-startled Christians, by their very nature and existence, say something to everyone involved. To the opposition, it says, "You are condemned. You are headed to destruction." To the Christians it is reassurance. When they are united together and un-afraid of the consequences of being a Christian, it is a sign of genuine salvation.