"Being confident of this very thing, that he which hath begun a good work in you will perform it until the day of Jesus Christ." (Philippians 1:6)
Our Lord gave this powerful promise to perform the good work that He began at and with the church at Philippi (the "you" is plural in the Greek text).
It is an earthly, temporal promise; that is, the promise is to "perfect" the good work "until the day of Jesus Christ." The church at Philippi closed its earthly doors centuries ago. Something much more than mere continuation is pledged.
Surely our Lord has in mind His assurance that "the gates of hell" would not prevail against His church (Matthew 16:18), but there were some churches to whom Christ spoke who were in danger of losing their "candlestick" or church-hood (Revelation 2:4; 3:16). What, then, can we be assured of by this marvelous promise?
Perhaps the basic "good work" that our Lord refers to is seen in the list of commendations given to the seven churches in the letters dictated to John at the beginning of Revelation. All except Laodicea had some strengths. Even troubled Sardis had a "few names" not yet sullied and "things which remain" that were still good and worth preserving (Revelation 3:1-4). Our Lord knows all His works "from the beginning" (Acts 15:18) and sees the eternal fruit of our ministry that ripples long beyond our short earthly life (Revelation 14:13).
There is also the mystery of our being "builded together for an habitation of God through the Spirit" (Ephesians 2:22)--a "spiritual house" that produces "spiritual sacrifices, acceptable to God by Jesus Christ" (1 Peter 2:5). All of this, perhaps, is what our Lord had in mind when He promised to perform the good work He had started in Philippi. HMM III