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. (Php 1:6) ESV
For a little bit of context Paul founded the church in Philippi on his 2nd missionary journey (Acts 16) sometime around AD 51. Historical data supports a primarily Gentile congregation that had been healthy from its conception but was presently struggling with a little disunity. The traditional view of the authorship and dating maintains Paul wrote to the Philippians around AD 60-62 from his first imprisonment in Rome. Paul previously visited Philippi twice during his third missionary journey, once in the beginning (2Cor 8:1-5) and once near the end (Acts 20:26). Listed among the “prison epistles” (Ephesians, Colossians and Philemon), Paul wrote to the Philippians with a minimum of 5 reasons. First, since the Philippians had sent Epaphroditus with a gift, he wanted to thank them (4:10-18). Secondly, he wanted them to understand his sending Epaphroditus back (2:25-26). Thirdly, he wanted them to know his circumstances (1:12-26). Fourth, he responded to their unity issue and lastly he wrote to warn them about false teachers (3:1-4:1).
Regarding our verse at hand there are a few distinctions that I think have significant value especially when seen in light of the whole of Scripture. Paul uses the same word for began, ἐνάρχομαι (enarchomai) that he uses in Galatians 3:3. Both times it is in reference to salvation and both times it is God who has began this process (Gal 3:3 states “having began by the Spirit”). This is significant because like 1 Corinthians 1:8, Ephesians 1:13-14; Jude 24, Hebrews 7:25; 12:2 to name but a few, God’s initiating work is always satisfied in His completing work. Should He begin a work that you have to complete, I’m not so convinced Jesus words on the cross “it is finished” would be accurate. As a believer in Jesus Christ and His finished work, this should give us great confidence in not only what He has done but in what He has promised to finish.
The only other point in this verse is do not confuse the “day of Jesus Christ” with the “day of the Lord”. The “day of the Lord” refers to final Judgment whereas the “day of Jesus Christ” “day of our Lord Jesus Christ” and “day of Christ” all point to final salvation, reward and the glorification of believers (see 1 Cor 3:10-15; 4:5; 2 Cor 5:9-10).