Philippians 1:6

6 Being confident of this very thing, that He who has begun a good work in you will complete it until the day of Jesus Christ.

Paul was confident that the Philippians’ fellowship in the gospel, which had lasted this long, would continue until the Rapture. This was the reason for Paul’s joy.

confident = reached a conclusion based on reasonable grounds. Paul was confident and would continue to be so (Greek tense)

He = God

good work — regeneration (past), sanctification (present) and glorification (future). What God begins, He will finish (1 Samuel 3:12; Ecclesiastes 3:14).

Day of Jesus Christ — beginning with the Rapture and continuing with the Lord’s presence with His people