In his commentary on Philippians, Gordon Fee pointed out that as many as 18 different suggestions have been made for the identity of the “opponent” in Philippians. In this case, the identity of the opponent may provide a bit of a hint to the date of the letter.
Paul begins to deal with these false teachers” in chapter three, although those who preach the gospel from impure motives in chapter one are likely the same group. One of the more common identifications of the false teaching is that they are Judaizers, similar or identical to those in Galatia. Certainly circumcision is an issue (3:2), and the fact that Paul boasts in his own credentials as a law-observant Jew might imply that his opponents have a similar boast. It is possible that these are Jewish teachers trying to re-convert the Jewish Christians or Gentile God-fearers trying to encourage gentile converts to join them in keeping the law.
If the book was written in the early 60’s from Rome, it is surprising that the issue of Gentile conversion is still a major issue. The issue seemed to be settled after Acts 15; for it to arise again nearly twelve years after the Jerusalem Conference seems unlikely. As such, this is a good argument in favor of the early date of the book, written from an Ephesian imprisonment (Polhill, P&HL, 166).
On the other hand, if Philippians was written from Rome in the early 60’s, it is only a few years before the outbreak of the war against Rome in Palestine. This was a time of extreme nationalistic pride and hopes for an independent Israel. If this period was anything like the Maccabean period, it is possible that circumcision for those within the Jewish covenant was emphasized. We are on the same sort of ground as Galatians (are the Gentiles converts to Israel)? If they are, then they must conform to the covenant and be circumcised. Even as far away as Rome, it is possible that Diaspora Jews saw the boundary markers of circumcision and food laws as non-negotiable for the Jewish people, including those who were ethnically Gentile and sought to align themselves with the Jewish Messiah Jesus.
Another suggestion which has merit is that of David deSilva. Based on his reading of Philippians as a “letter of friendship” he has suggested that the opponents of Paul in the letter are not actual opponents in the church, but rather Paul is “using a common strategy for building up unity and cooperation within a group: warning about the presence of hostile and dangerous groups on the outside, against whom the Philippians need to present a united front” (deSilva, “No Confidence In The Flesh” Trinity Journal 15:1 (Spring 1994): 31-32)
On balance, I tend to agree with Polhill and date the letter early. While the evidence for an Ephesian imprisonment is thin, there is enough to lead to me believe Philippians at least was written from Ephesus in the mid 50’s rather than Rome in the early 60’s. (The other prison epistles were written from Rome, but that is for another time!)