"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" (Philippians 1:6).
his is one of the most basic truths of Christianity: Salvation is not a work the sinner does for God; it is a work God does for the sinner. Ephesians 2:10: "We are his workmanship."
Even the good works we do as Christians are the result of God's work in us. Those good works are not accomplished by our own willpower or initiative. Ephesians 2:10 continues thussly: "For we are his workmanship, created in Christ Jesus for good works, which God prepared beforehand, that we should walk in them."
If God foreordained even the good works we do, and since He is the one who empowers us both to will and to do them (Philippians 2:13), then salvation is truly all God's work.
And He always finishes what He starts. That's the point of the verse at the top of this post.