By living a worthy life, the church will stand firm in one spirit (v. 27-28). One’s “manner of life” (πολιτεύομαι) refers to being a good citizen. If someone was a Roman citizen, there were a number of expectations for proper behavior in the public forum. This refers to both a legal responsibility as well as conduct in public. By analogy, a “good citizen” in America pays their taxes and votes in elections, properly registers and insures their car, etc. You cannot call a person who refuses to pay taxes, breaks the Law regularly, or runs around burning American flags a “good citizen.”
“Manner of life” can be used as a metaphor for living in accordance with the Law. 3 Macc 3:4, for example, describes the way of those Jews who had kept themselves separate with respect to foods, but had gained a good reputation for various good works. But these differences were so significant that they fell under suspicion as “hostile and greatly opposed to the government” (3 Macc 3:7) and eventually the government oppressions the Jews because their “manner of life” was so different than the Greeks in Egypt (3:11-30). The same sense of the word appears in 4 Macc 2:8 where one whose “manner of life” conforms to the Law stands in contrast to a number of typical vices. Josephus refers to keeping the Law, but also paying the Temple tax and other civic duties (Ant. 12.142). The word appears in other Jewish literature to describe proper conduct of life with respect to the Law. It is not insignificant that the Jews in 3 and 4 Maccabees were perceived as hostile to their culture and were persecuted for their “manner of life”
To have a manner of life “worthy” of some ideal is a common way of expressing the goal of spiritual life in the New Testament. Perhaps this might be thought of as “live up to an expectation.” For Roman citizen, the expectation is to live like a Roman citizen should; for the one who is “in Christ,” they are to live worthy of the Gospel! Paul begins the second half of Ephesians with similar words (“walk in a manner worthy of the calling”); in 2 Thess 2:12 he encourages his readers to “walk worthy of God” (cf. 3 John 6); in Col 1:10, it is “walk worthy of the Lord;” in Rom 16:2, it is “walk worthy of the saints.” Deissmann reports this word was used on inscriptions in Pergamum (Biblical Studies, 248). Athenaios, a priest of Dionysus and Sabazius, is extolled as “worthy of god.” Whatever these priests did, they were considered good examples for other worshipers.
The goal in Philippians 1:27 is the Gospel of Christ. The one who is “in Christ” is not a citizen of Rome. Nor should they conform their lives to the Law quite like the martyrs in 3 and 4 Maccabees. Their loyalty is to the Gospel of Christ only. Everything the individual Christian or local church does ought to be viewed through the grid of the Gospel.