Paul is still not done with his thanksgiving. His mention of their participation in the gospel from the first day until now leads him to add, in a somewhat digressive way, that he is fully confident that what was true from the first day and is still true now will be true at the end as well. But this confidence has very little to do with them and everything to do with God, who both began a good work, which is still in evidence, and will carry it on to completion until the day of Christ. Thus, having reminded them of his joy over their good past and present, Paul turns now to assure them of their certain future.
The good work that God has begun and will bring to full fruition may very well include their grace of giving, or perhaps their continued participation in the gospel in every way. More likely, however, it refers to God's good work of salvation itself, of creating a people for his name in Philippi. If so, the sentence anticipates 2:12-13, where Paul urges them to keep working out their common salvation in the way they live together as God's people in Philippi, since God is at work in them both to will and to do for the sake of his own good pleasure. Thus the concern is for their participation in the gospel in yet another sense, not so much their sharing it as their experiencing it and living it out in Philippi.
The day of Christ, on which God will bring his work in them to completion, points to the final consummation of salvation at Christ's (now second) coming. The reason for this otherwise digressive clause is probably related to another concern that surfaces at several points in the letter: that some of them have apparently begun to lose the basic future orientation that marks all truly Christian life. In 3:15-17 they are urged to follow Paul's example of desiring to know Christ above all (3:6-11) and eagerly to pursue the prize of knowing him fully at the end (vv. 12-14). Here Paul anticipates that exhortation by focusing on God's own commitment to bring them to completion on the day of Christ.
Believers in Christ are people of the future, a sure future that has already begun in the present. They are citizens of heaven (3:20) who live the life of heaven, the life of the future, in the present in whatever circumstances they find themselves. To lose this future orientation, and especially to lose the sense of "straining toward what is ahead, . . . toward the goal to win the prize for which God has called [us] heavenward" (3:13-14), is to lose too much. Thus their present gift, which also reminds Paul of their long association in the gospel, leads him to digress momentarily, to remind them that even in the midst of present difficulties, God has in Christ guaranteed their future as well as blessed their present situation in Philippi.
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