There is a lot going on in these few verses. Paul begins by calling the Philippians once more to rejoice, a recurrent theme throughout this letter so far. But we are surprised — jarred — by what we encounter next, a denunciation of false teachers that is completely unexpected. Though it comes without warning, this change of tone makes perfect sense in light of both the grave danger a false gospel presents and Paul’s passion to see the Philippians contending together as one for the gospel — the true gospel.
