The opening clause of the prayer is followed by two purpose clauses. One (item 3) expresses immediate purpose, why they need ever-increasing knowledge and moral insight—so that you may be able to discern what is best. The other (item 4) expresses his ultimate concern for them—so that they may be pure and blameless until [better "for"] the day of Christ.
For a truly Christian life, some things matter and others do not. In light of the term fruit of righteousness in verse 11 and the contrast in 3:1-11 between righteousness in terms of lawkeeping and that which is through faith in Christ, this petition probably anticipates the warning and personal testimony of that passage. Neither circumcision nor uncircumcision counts for a thing, Paul says elsewhere (1 Cor 7:19; Gal 5:6; 6:15). What counts, rather, is keeping the commandments of God (1 Cor 7:19), which in Galatians 5:6 is interpreted as "faith expressing itself through love." This is the kind of insight he prays for them to have, so that they will be able to continue to discern what is best.
The ultimate purpose for the preceding concerns, that they might be pure and blameless for the day of Christ, reflects the urgency already voiced in the thanksgiving that they be found complete at the coming of Christ. Pure most likely refers to purity (sincerity) of motive, in terms of relationships within the community. Likewise, the word translated blameless is not Paul's regular word for this idea. Ordinarily, as in 2:15 and 3:6, he uses a word denoting behavior that is without observable fault. But this word suggests being blameless in the sense of "not offending" or not causing someone else to stumble.
This choice of words probably reflects the present situation in Philippi. The behavior of some appears to have the latent possibility of mixed motives, or at least is a potential source of offense. Paul prays that they may stand blameless on the day of Christ, not having offended others through equivocal behavior.
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