Earlier in Philippians 3, Paul spoke words of warning against Judaizers, legalists who believed that righteousness was found in works. Now, Paul exposes the danger of the Antinomians, those who sought to live free of any moral restriction. His point was that either extreme is incorrect. The saved are under law—the law of agapè (John 15:12). The saved are free—free to love with the love of Christ (Galatians 5:13). In either case, the response to the free salvation that Christ extends to us is agapè.
This response takes commitment to knowing Christ at an incremental level—moment by moment—and Paul understands this. It takes a day-by-day denial of demonic temptation—and Paul understands this.
Paul says that knowing Christ is his singular goal. Not to receive salvation, which has already been secured, but to receive the reward of knowing Christ in as intimate and personal a way as is possible in this life.
Do we want to know Christ on that level? Sadly, for many a casual knowledge of Christ is sufficient. We desire to know Him as Savior, but we do not want to submit to Him as Lord. We want the benefits of salvation, but we lack the commitment to live sanctified (spiritually set apart) lives. And we miss out on so much because we lack that commitment.
The fact that we are saved is entirely due to the work of Jesus Christ. But the reward of our salvation is the ability to know our Savior. It is a reward that is dependent upon us and our focus on Him as the Source of our being.