Philippians 1:6 (ESV) …. “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.”
Great confidence gripped the apostle as he thought and prayed for the Philippians. The perfect tense of the Greek word translated being confident indicates that Paul had come to a settled conviction earlier and that he still was confident it was true. What was he so confident and sure of? It was that God would most certainly continue on to completion the good work He had begun in them. That good work was their salvation. It may also have included their fellowship and sharing of their bounties with Paul.
Paul had no doubt that God would continue in the Philippians what He had begun to do in them. God would work in them until the day of Christ Jesus. In 2:16 Paul called this “the day of Christ.” Though Paul did not know when that day would occur—when all believers would be caught up to meet the Lord in the air—he did know that God would continue the work He had begun in His own children.[1]
[1] Lightner, R. P. (1985). Philippians. In J. F. Walvoord & R. B. Zuck (Eds.), The Bible Knowledge Commentary: An Exposition of the Scriptures (Vol. 2, p. 649). Wheaton, IL: Victor Books.