Paul's thanksgiving is not finished. Verses 3-4 focused on the Philippian believers themselves and Paul's joy in remembering them in prayer. He now indicates the basis of his joy, which serves as further reason for his thanksgiving: their partnership (koinonia) in the furtherance of the gospel.
The precise nuance of koinonia in this clause, however, is not easy to pin down. Usually translated into English as "fellowship," the word primarily refers to participating in something rather than sharing something in common with others. Its basic sense here, then, is "participation in the spread of the gospel," which in light of verse 7 very likely carries the further connotation of doing so in "partnership with Paul."
It does not take much reading of Paul's letters to recognize that the gospel is the singular passion of his life; that passion is the glue that holds this letter together. The gospel, especially in Philippians, for Paul refers primarily neither to a body of teaching nor to proclamation. Above all, the gospel has to do with Christ, both his person and his work. To preach Christ (vv. 15-16) is to preach the gospel, which is all about Christ; to preach the gospel is to proclaim God's good news of salvation that he has effected in Christ. Thus Paul's joy in prayer is prompted by their partnership in [the furtherance of] the gospel.
The present focus is on the Philippians' longtime association with Paul in the gospel, from the first day until now. According to the rest of the letter this took place in two ways: first by their sharing with him of their material means as he is imprisoned for the sake of the gospel (4:15-16); second by their proclaiming, and living in keeping with, the gospel in Philippi, where they are urged to "contend as one [person] for the faith of the gospel" (1:27) as they there "hold out the word of life" (2:16).
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