If the church is living their lives worthy of the Gospel, they will be striving together for the gospel and not frightened by any opposition they face. The verb Paul uses here is cognate of ἀθλέω, “to compete in a contest,” implying strenuous action. The book of 4 Maccabees 17:14ff uses this word group to describe martyrs, and the cognate (ἄθλησις) appears in Hebrews 10:32 to describe the hard struggles of the church at Rome as they were persecuted by civil authorities. By the end of the first century, Clement describe the apostles who had given their lives for the gospel as “contending to the death” (1 Clem 5:2). Paul uses a form of the word stressing the unified action of a team (συναθλέω) with a prefix which is something like the English prefix co-. They are “co-strivers,” hence the ESV’s “side by side.” The related noun (συναθλητής) refers to a fellow athlete (LSJ).
The church does not have reason to be frightened by their suffering. The verb Paul chooses to use in this verse (πτύρω) is not the usual word for fear in the New Testament. While it can be translated “terrified,” it is better translated “shy.” In secular Greek it was used of horses that were shy, easily frightened, etc (D.S.2.19). Since it always appears in the passive, and in this case the agent of the passive verb is “opponents,” it is probably best to translate this “do not let yourself be intimidated by your opponents.”
Why would the Philippians be intimidated? They are a tiny minority with a view of the world that is radical in Greco-Roman. They have no temple, priesthood, or sacrifice. They do not worship the gods in Philippi, nor do they even recognize their existence! They worship in homes, sharing food and fellowship with people of various social classes. Remember the photograph of the man in Tienanmen Square. One man stood in front of the tanks, refusing to move. This one man stood alone against an ultimately powerful force.
The Philippian church is something like that man, a single example of a Christian community in the vast work of Roman Philippi. Paul’s encouragement to Christians at Philippi is to live a life consistent with the Gospel. In the context of Philippians, this means unity of heart and mind as well as willingness to suffer for the sake of the Gospel.
In fact, by living a worthy life, the church will suffer for their faith (v. 29-30). Contrary to public expectations of success, a life worthy of the Gospel will lead to conflict with culture, resulting in suffering. A Roman would compete for honor, but he expected to win! Paul says here that even if the church is doing everything right, they are going to suffer loss. This is the same conflict Paul is fighting. He is in chains on account of the Gospel of Christ, yet he has already described this “loss” as a “gain.”
“The Pew Forum on Religion and Public Life finds that Christians are suffering persecution in more places today than any other religious group; between 2006 and 2012, Pew says, they were targeted for harassment in 151 countries— three-quarters of the world’s states” (Scot McKnight’s Jesus Creed, see also Paul Marshall’s original story at The Weekly Standard). While it is hard to look at suffering of innocent people as a gain, from the perspective of Philippians 1, people suffering on account of their faith in Jesus Christ is a victory.
Paul’s encouragement to life a life worthy of the Gospel anticipates his description of Jesus in chapter 2. Just as Jesus humbled himself and took on the form of a servant, so too the church at Philippi must humbly serve others even if that means suffering loss of honor in Roman Philippi.