"For I am sure that He who started this good work in you will carry it through until it is finished on the day of Christ Jesus."
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After one comes to believe the Lord Jesus Christ has saved you by His perfect life and death for your sins, has risen, has gone to the "Father's House" and prepared a place for you and you have this perfect peace ("My peace I give to you" - perfect peace of the perfect man granted to hearts that know they don't deserve it but receive it by unmerited grace of God) you still sin. This brings a discouragement that must be tempered with hope. This passage gives that hope. It is HE who started the good work in you (regeneration - the new birth - birth of a heart that desires to please God out of gratitude for what He has done by saving you through the perfect life and the subsitutionary death of His Son) He will not be done with you until He has finished the work of perfecting you. He 'will carry it through' this work of sanctification. So do not be discouraged, believe He is working in you especially when you have fallen again and sinned. He will discipline you. It won't be pleasant. No discipline is pleasant at the time. But He will strike you when you sin and He will do so in such a way as to give you the taste of His tough love. He won't let you fall away from Him all the way but will pull you back with His Shepherd's staff, perhaps after knocking you with His Shepherd's rod. But however He brings you back to Himself it is for your good and it is because He will carry on the work He has started in you to perfect you.
That perfection will not be complete until the DAY OF CHRIST JESUS. That is, the day when you die and in respect to your soul you will be perfected in all motives and desires and the day when your body itself is made holy through the resurrection and remarkable transformation from its ruin into a body fit for your perfected soul. The day of Christ Jesus is that day when He perfects the work He has started in you and it will come just as surely as He will come again to establish His Kingdom and remake the heavens and the earth in their perfection as well. He has told us "you must be perfect, even as your Father in Heaven is perfect" and He has promised He will not stop carrying on His work in us until He works His own perfect life into us so thoroughly that we too enjoy that perfection He requires. This means we who believe in Him will not "rest" in the work He has already done but will find ourselves daily repenting of sin and that which is imperfect and yet daily find Him improving us for Heaven. The rooms in need of improvement in our house are plentiful but we know the Carpenter of Nazareth, when He starts a work in one, doesn't stop until He has perfected that work. By the power of the Holy Spirit we who believe in His death for our despicable sins deny ourselves to have any good thing in ourselves yet claim Him and His righteousness as our own by His grace.
John Calvin's Commentary on this verse
This provided here thanks to this website: http://www.ccel.org/c/calvin/comment3/comm_index.htm
It is asked, however, whether any one can be certain as to the salvation of others, for Paul here is not speaking of himself but of the Philippians. I answer, that the assurance which an individual has respecting his own salvation, is very different from what he has as to that of another. For the Spirit of God is a witness to me of my calling, as he is to each of the elect. As to others, we have no testimony, except from the outward efficacy of the Spirit; that is, in so far as the grace of God shews itself in them, so that we come to know it. There is, therefore, a great difference, because the assurance of faith remains inwardly shut up, and does not extend itself to others. But wherever we see any such tokens of Divine election as can be perceived by us, we ought immediately to be stirred up to entertain good hope, both in order that we may not be envious 15 towards our neighbors, and withhold from them an equitable and kind judgment of charity; and also, that we may be grateful to God. 16 This, however, is a general rule both as to ourselves and as to others -- that, distrusting our own strength, we depend entirely upon God alone.
I also found the below at:
The word or doctrine of assurance can be misleading. A believer can have
eternal security and not realize it or believe it. This is the reason some
continually come forward during a service or continually tries to earn their way
into heaven by doing good works. They are just not sure that their salvation
"took." There are several possible reasons for sometimes doubting.
Some doubt their commitment to Christ, because they cannot seem to stop
sinning. All sin! Paul 205
said he could not help himself. They have taught some in their churches that
they can fall from grace, but this is not what we find in Scriptures. Some lack
the peace of assurance because of the procedure they went through when they
accepted Christ. They keep asking themselves questions. Am I really saved? I did
this privately, was that all right? Did I ask in the right way? Can God really
love me since I am still sinning? God has promised that He will keep you and
make you a new creature. 206
Some think they must have a deep emotional experience with bells and lightening
and if they don't, they have doubts. People are either saved or lost, they may
not be able to pinpoint the time or a given moment they were converted, but we
all grow in our understanding of conversion. God knows the exact second even if
we do not.
When a serious sin or a tragic experience occurs in the believer's life, then doubt sometimes accompanies such an experience. We need to realize that Christians will sin, and that sin does not cause us to lose our salvation. The Christian is never sinless, 207 for "we all stumble in many ways" (NIV) and sin does not cause a forfeiture of salvation. Eternal Security does not give us the license to sin and never excuses sin, but through sin, doubt and repentance the Christian will also grow in holiness.
We have known people who made a commitment to Christ and were on fire for the Lord only to repudiate their faith later. We have heard of great Christian leaders who have committed gross sins and destroyed their ministry. We know that Christians "backslide." We know that Christians can and do in fact commit gross and heinous sins. True Christians can fall, but we do not believe that they can fall totally and finally. King David was guilty not only of adultery but murder. His subsequent repentance was as deep as his sin. David sinned terribly but not totally and finally. He was restored. Both Peter and Judas betrayed Jesus, but Judas killed himself and Peter repented and was restored, becoming a leader in the church. What made the difference between these two men's action after they betrayed Jesus? Jesus predicted that both of them would betray him. Judas was a son of perdition from the beginning. His conversion was spurious. Jesus did not pray for his restoration, but He did for Peter's. 208 All three of these words really mean the same thing, that the true believer will not lose his salvation or his eternal life.
Arminianism teaches that God's decrees are based on His foreknowledge. Election was due to foreseen faith and sin to foreseen resistance to grace. Arminius also viewed foreknowledge as foresight, but he did say that "God decreed to save some and damn certain particular persons." 209 Arminianism clearly teaches that a believer may lose his salvation. Arminius said: "I never taught that a true believer can either totally or finally fall away from the faith and perish; yet I will not conceal that there are passages of Scriptures which seem to wear this aspect.