Adam Clarke on Philippians 1:28

In nothing terrified by your adversaries - So it appears that the Church at Philippi was then under persecution.

Which is to them - Ἡτις αυτοις εστιν. Some very judicious critics consider ἡτις as referring to πιστις, the faith of the Gospel, which they, the heathen, considered to be a token of perdition to all them who embraced it; but, as the apostle says, it was to them the Philippians, on the contrary, the most evident token of salvation; for, having embraced the faith of our Lord Jesus Christ, they were incontestably in the way to eternal blessedness.