Let your conversation... — The original is here (as in the famous passage, Philippians 3:20), Use your citizenship (that is, of the kingdom of heaven) worthily of the gospel of Christ. The same word is employed by St. Paul in Acts 23:1 (“I have walked in all good conscience before God”), with an obvious reference to his citizenship in the chosen nation of Israel. Its use in this Epistle is suggestive — both as natural to one contemplating the great imperial city, and writing to the people of a Roman colony proud of their full citizenship, and also as leading on to that great conception of the unity of the Church in earth and in heaven, which is the main subject of the Ephesian, and in some degree of the Colossian, Epistle.
In one spirit, with one mind. — Rather, in one spirit, one soul. The phrase “in one spirit” may refer to the spirit of man, or to the Spirit of God. If it be intended to be strictly parallel to the “one soul” (which has no separate preposition in the Greek), the former sense is manifestly suggested. If, however, the words “with one soul” be connected, as is not unnatural, with “striving together,” this suggestion falls to the ground; and the usage of this Epistle (see especially Philippians 2:1-7), and the other Epistles of the same period (Ephesians 2:18-22; Ephesians 3:5; Ephesians 5:18; Ephesians 6:18; Colossians 1:8), certainly favours the latter interpretation. In either case “the soul” (as in the famous three-fold division of men’s nature in 1 Thessalonians 5:23) is that element of humanity which is the seat of emotion and passion. (Comp. the “one heart and one soul” of Acts 4:32.) This element the Christianity of the New Testament, unlike Stoicism or asceticism, will not crush, but enlist, as it enlists the body also, in the free service of God.
Striving together for the faith. — Properly, with the faith. The faith of the gospel — the power of Christianity — is personified. The Philippians are to be combatants on the same side against the same foes (compare the use of the same word in Philippians 4:3). The metaphor seems drawn from the games, as is seen by the use of the simple verb in 2 Timothy 2:8, “If a man strive... he is not crowned, except he strive lawfully.” In the exhortation to stand fast (comp. Ephesians 6:13-14) we have the element of passive endurance, here of active and aggressive energy.