What does Philippians 3:16 mean?

Verse 15 set a lofty goal: putting the pursuit of Christ above all things. Paul calls on all Christians to seek fellowship with Jesus first, and to treat every other concern as "rubbish." Verse 16 seems to be something of a concession, to encourage those who know they are not walking in that path, yet. At the very least, his readers were not to fall back into previous sinful practices. This is part and parcel of his runner's analogy: move forward and focus on Christ. As a minimum, he expected believers to not lose progress in following Christ. In terms of his racing example, Paul is encouraging the Philippians to not lose ground, even if they don't feel they're gaining it.

An interesting connection is also seen with the words translated "attain" in verse 11 and here. In verse 11, the Greek word is katantēsō, in verse 16, it is ephthasamen. Both involve the concept of achieving, accomplishing, or arriving. In verse 11, Paul's focus was to attain "the resurrection of the dead," referring to his future with Christ. In verse 16, the focus is on holding on to what has already been attained. Though not the main point of this passage, Paul connects with the idea of believers who have confidence in their salvation, a theme he and others address elsewhere in the New Testament (Romans 8:37–39; 1 John 5:13).