The apostle Paul had just talked about Jesus’ obedience on the cross in verse 8:
“And being found in appearance as a man,
he humbled himself
by becoming obedient to death—
even death on a cross!”
The Philippians are commended by Paul for their obedience, when he was with them and when he wasn’t with them anymore. We might have all encountered instances where one would obey a command or law when an authority figure (policeman, a traffic guard, a parent…) was around, but was quick to disobey when there was no one there to hold you accountable (mask or no mask, jaywalking, red light/green light, don’t eat that cookie, you name it!).
Paul’s encouragement is for people to continue to work out their salvation with fear and trembling, not to earn it by what they do, because Jesus’ work of atonement was fully done on the cross, but for each one to be actively willing to be transformed to the likeness of Jesus and aware of the work of the Holy Spirit in their lives. It is from the cross that the work of sanctification is carried out, by the help of the Holy Spirit!
“Am I prepared to let God grip me by His power and do a work in me that is worthy of Himself? Sanctification is not my idea of what I want God to do for me; sanctification is God’s idea of what He wants to do for me, and He has to get me into the attitude of mind and spirit where at any cost I will let Him sanctify me wholly.” – Oswald Chambers