Paul mentions this as another reason for remembering the Philippians with gratitude and joy in his prayers. He is convinced that God will continue the good work he started in Philippi. That good work
is primarily their repentance and faith. This resulted in them cooperating in the gospel. That is what God has accomplished. God is at work; therefore, Paul can work and the Philippians can help. God will also maintain his work and bring it to a successful conclusion. The completion comes on the day of Christ Jesus. That is how far God’s work goes which began so small in Philippi. On the day Christ returns, it will be seen how the Philippians have cooperated in the great work of God.
6 And I am sure of this, that he who began a good work in you will bring it to completion at the day of Jesus Christ.