Therefore, my beloved, as you have always obeyed, so now, not only as in my presence but much more in my absence, work out your own salvation with fear and trembling, for it is God who works in you, both to will and to work for his good pleasure. The Apostle Paul is not suggesting the the Philippians should try to earn their salvation by doing good works. Once a person has received the gift of God's salvation, then it must be lived out through the realities of daily life by continual obedience to God. Each believer in Christ progressively comes to experience all aspects and blessings of salvation:
"Not that I have already obtained this or am already perfect, but I press on to make it my own, because Christ Jesus has made me his own" (Philippians 3:12). Paul says that it is actually God's grace and presence in the Holy Spirit working in you that enables and empowers you to live a responsible and productive Christian life that accomplishes God's will.
