Philippians 3:9-11 "not having a righteousness of my own derived from the Law, but that which is through faith in Christ, the righteousness which comes from God on the basis of faith, that I may know Him and the power of His resurrection and the fellowship of His sufferings, being conformed to His death; in order that I may attain to the resurrection from the dead."

Reading this passage may make one think of the Roman Catholic idea of justification: that the righteousness of God is infused into us, producing good works that are meritorious before God and qualify us for heaven.  Roman Catholics may cite verse 12 which says "Not that I have already obtained it or have already become perfect, but I press on so that I may lay hold of that for which also I was laid hold of by Christ Jesus."  A typical explanation by a Roman Catholic would be "See, Paul didn't have assurance of his own salvation!  He was pressing on, trying to obtain it!"

Besides the fact that the "it" here is not salvation, we will set this passage aside for a quick moment and look at other references in Philippians that can help us shed light on this.

Philippians 1:6 "For I am confident of this very thing, that He who began a good work in you will perfect it until the day of Christ Jesus."

Paul highlights the God-centered work of the gospel, it is God who began a good work and it is God who will perfect it until the day of Christ's return.  If God is who keeps us, why would we not have assurance of our salvation since it is God who saves?  A man-centered or synergistic view of the gospel misses this important fact!

Philippians 1:21-23 "For to me, to live is Christ and to die is gain. But if I am to live on in the flesh, this will mean fruitful labor for me; and I do not know which to choose.  But I am hard-pressed from both directions, having the desire to depart and be with Christ, for that is very much better."

Here we see that Paul, contemplating the matter of living and dieing, had the desire to depart and be with Christ.  There is no mention of this being a possibility (that he might be with Christ), but that he would be with Christ.  He has assurance that if he was to die, he would be with Christ.

Now heading back to the original passage.  Starting in chapter 2 and continuing into chapter 3 and up to verses 9-11, Paul is urging the believers to live a life worthy of the gospel, and the example of Christ demonstrates this perfectly.  When Paul says "not that I have already obtained  the resurrection of the dead or have become perfect" (verse 12), he is stating the obvious.  He has not died and been resurrected, nor has he become perfectly righteous.

It's similar in thought to 1 John 3:2-3 where having a hope to be transformed and be rid of our sin nature, we purify ourselves, looking forward to the day when we are made perfect by Christ.

It's confirmed by Philippians 3:20-21 which says "For our citizenship is in heaven, from which also we eagerly wait for a Savior, the Lord Jesus Christ; who will transform the body of our humble state into conformity with the body of His glory, by the exertion of the power that He has even to subject all things to Himself."

Paul is not striving so that he might obtain salvation, he is striving to be perfected by Christ; looking forward to the day of Christ's return and the resurrection of the body where he will be transformed into conformity with the body of His glory. 

This is the hope and practise of all true believers.  Those who are saved by faith have the assurance of a resurrected body, with this hope in mind we purify ourselves and strive toward that goal, looking forward to the day of our Lord.  Not already having been made perfect, but our salvation is sure.

PS. In Philippians 3:12 "but I press on so that I may lay hold of that for which also I was laid hold of by Christ Jesus."  Notice how Paul phrases this:  he presses on so that he may lay hold of that (the resurrection/conformity to Christ - vv 10,11) and it was for this purpose (the resurrection/conformity to Christ) that he was laid hold of by Christ Jesus.  Again we see the God-centeredness of the gospel.  Jesus laid hold of Paul for the purpose of resurrecting him, conforming his body to His own glorified body.

How can we not have assurance in our salvation since it is the very work of God?!