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.Paul is confident, but he is not confident in the Philippians or their abilities. He is not assured because of their outstanding character or their past successes. Paul's confidence in this ongoing work of salvation is based entirely in the one who began the work. The one who began the work is the one who will complete the work.
(Philippians 1:7, ESV)
This is hugely important to understand. So many questions about salvation and those connected issues like: eternal security, losing your salvation, walking away from the faith, predestination, election, progressive sanctification, and confidence in salvation... to understand what the scriptures say about the actual salvation process and who is really at work resolves many of these issues.
Consider this Commentary on Ephesians 2. In this passage, we can read that we are truly dead in our sin, "But God, being rich in mercy, because of the great love with which he loved us, even when we were dead in our trespasses, made us alive together with Christ—by grace you have been saved..." (Ephesians 2:4-5)
He goes on to say, in one of the most quoted passages of the New Testament, "For by grace you have been saved through faith. And this is not your own doing; it is the gift of God, not a result of works, so that no one may boast." (Ephesians 2:8-9) Did you catch that? Even the faith, which most of us consider to be "our part" Paul says, "And this is not your own doing."
For some this might seem confusing, but understand that good Bible interpretation is rooted in the attempt to simply say the same things that the Bible says, and not to always answer who these things meet in eternity. What we can be sure of, is that he who began this work, will complete it. "For we are his workmanship, created in Christ Jesus for good works, which God prepared beforehand, that we should walk in them." (Ephesians 2:10)
So we work like our lives depend on it, so "that by any means possible I may attain the resurrection from the dead." (Philippians 3:11)And this is a challenging, fearful work that you "... work out your own salvation with fear and trembling..." (Philippians 2:12b) But we can rest assured, "... for it is God who works in you, both to will and to work for his good pleasure." (Philippians 2:13)