"Finally, my brothers, rejoice in the Lord! It is no trouble for me to write the same things to you again, and it is a safeguard for you." NIV translation
Paul assures the Philippians that he doesn't mind reminding them of the things that they have already been taught. He says, in fact, that it is necessary for their own protection from false teachings.
Philippians 3:2 & 3
"Watch out for those dogs, those men who do evil, those mutilators of the flesh. For it is we who are the circumcision, we who worship by the Spirit of God, who glory in Christ Jesus, and who put no confidence in the flesh -" NIV translation
Now, Paul gets specific about the things that these men were teaching and once again it comes down to law versus grace. He calls them dogs just as the prophet Isaiah did (see Isaiah 56:10) in referring to prophets & teachers that just told the people what they wanted to hear instead of the word of God. You see, people want to hear that they can do something towards their salvation and so these false teachers would add a few rules to grace. The same sort of thing happens even today within the body of Christ as we see all kinds of denominations within the church. Most of these divisions (denominations) are based on some point or other of the law. Most pastors do not want to tell the people that they are free in Christ but instead want to control them through some set of rules and traditions.
Philippians 3:4 & 5
"though I myself have reasons for such confidence. If anyone else thinks he has reasons to put confidence in the flesh, I have more: circumcised on the eighth day, of the people of Israel, of the tribe of Benjamin, a Hebrew of Hebrews; in regard to the law, a Pharisee;" NIV translation
Paul begins to list seven things that could have given him confidence in the flesh. He does this as sort of a my religion versus yours comparison. He is saying that, if there is anyone that thinks they are all that with regards to the law, they probably cannot even match his qualifications much less the standards of God.
The first thing that he mentions is the fact that he was circumcised on the eighth day according to the law. This tells us that he came from a religious family who raised him according to the Mosaic Law. The second thing that he lists is the fact that he is a full blooded Jew unlike many of the Judaizers. The third thing he lists is the fact that he is not just from any tribe but was descended from Benjamin who was the son of Jacob's right hand. The fourth qualification that he lists is the fact that he was a part of the religious system and the fifth is that he was a Pharisee who was considered the best of Israel.
"as for zeal, persecuting the church; as for legalistic righteousness, faultless." NIV translation
Paul's sixth qualification speaks of the fact that he was not content just to run the followers of The Way out of Jerusalem and quit like the rest of the religious leaders. He was, in fact, pursuing them to Damascus when he encountered Jesus. His final qualification speaks to the fact that he had brought all of the required sacrifices to the temple.
"But whatever was to my profit I now consider loss for the sake of Christ. What is more, I consider everything a loss compared to the surpassing greatness of knowing Christ Jesus my Lord, for whose sake I have lost all things. I consider them rubbish, that I may gain Christ, and be found in him, not having a righteousness of my own that comes from the law, but that which is through faith in Christ - the righteousness that comes from God and is by faith." NIV translation
Paul says that all of his religion is like trash compared to having his relationship with Jesus Christ. He had been one of the top of the heap as far as the religious system was concerned but his position in the system did not bring him righteousness with God.
The same thing is true today even in "the Church" as many attend regularly and go through the motions, traditions and rituals without faith in Jesus. If we compare our actions with that of Paul, we even come up short and he said that all of his qualifications were as trash. Paul lost his religion and we need to as well because it does not have the power to save.
Philippians 3:10 & 11
"I want to know Christ and the power of his resurrection and the fellowship of sharing in his sufferings, becoming like him in his death, and so, somehow, to attain to the resurrection from the dead." NIV translation
In this passage, we see Paul's desire to be like Christ in all things. As we see, Paul did not completely understand how all of this happens but the desire of his heart was to follow in the steps of Jesus. This speaks of what is known as sanctification.
Philippians 3:12 & 13a
"Not that I have already obtained all this, or have already been made perfect, but I press on to take hold of that for which Christ Jesus took hold of me. Brothers, I do not consider myself yet to have taken hold of it." NIV translation
Paul stated two times that he has not achieved the goal of being Christlike. He says that he continues to strive for that goal (even though he was under arrest in Rome at the time).
There are those who miss his point and take this to mean that, if we do not work at it, we will fall away from the faith and lose our salvation. That is not what Paul was saying as he was not pressing on to achieve salvation through his works but was seeking to be more like his Savior because of his salvation. This passage speaks of sanctification and not salvation. Salvation happens the moment we accept Jesus Christ as our Lord and Savior. Sanctification is a process that continues until we go to be with Him.
Philippians 3:13b & 14
"But one thing I do: Forgetting what is behind and straining toward what is ahead, I press on toward the goal to win the prize for which God has called me heavenward in Christ Jesus." NIV translation
How do we run this race that Paul speaks of? He gives us the answer here as he says that first he forgets what is behind. This speaks of forgetting our past failures in listening to the Spirit of God as He leads us through this process of sanctification. This is important because, if we dwell on our past failures, we will become discouraged and ineffective which is not of God.
The second thing we do is to strain toward the goal meaning we actively look for and listen to the voice of the Spirit. We expect God to lead us through everything instead of just going to Him when we feel that we need a little help.
So, what is the "prize" that Paul speaks of? There are many who confuse this with salvation but that is not what he was talking about. We see the answer to this question from Jesus' own words in the Parable of the Talents (Matthew 25). We see that the man who took what was given to him by his master and made it grow was given three things and we are reminded that the number three is associated with the earthly display of God's will. First, the man was rewarded by hearing his master say "Well done, good and faithful servant!". Personally, I think that hearing those words from my Lord will be the greatest reward of all. The second thing that the man received was that he was put "in charge of many things" and this speaks about what we are going to be doing on the New Earth when Jesus returns. The third thing that the man got was to "share your master's happiness" and this speaks to the fact that we are going to be able to see the fruits of our labor on this earth. We are going to know those that we have brought to faith in Jesus Christ and we will get to join with Him in welcoming them into His presence.
Philippians 3:15 & 16
"All of us who are mature should take such a view of things. And if on some point you think differently, that too God will make clear to you. Only let us live up to what we have already attained." NIV translation
Paul explains that this teaching is for those who are mature in their faith meaning that they are firmly grounded in the knowledge of the grace of God through Jesus Christ. He also trusted God to show the truth to those who were less mature and disagreed. He urges the Philippian believers to simply strive to live like Christ as He has saved us and even given us the power to do so through the Holy Spirit.
"Join with others in following my example, brothers, and take note of those who live according to the pattern we gave you. For, as I have often told you before and now say again even with tears, many live as enemies of the cross of Christ. Their destiny is destruction, their god is their stomach, and their glory is in their shame. Their mind is on earthly things." NIV translation
Paul reminds the believers that it is important to have the "right" friends. He tells us to join with those who are walking the talk as he did. We see that it made Paul sad that there were those who would not accept the gospel. Many times, I have heard that we must hang with the lost in order to earn the right to share the gospel with them but Paul tells us the opposite thing here. We are to share the gospel with people and trust God to use it to change their hearts. This is what Jesus was speaking of when He compared the kingdom of God with a man scattering seed (see Mark 4).
Philippians 3:20 & 21
"But our citizenship is in heaven. And we eagerly await a Savior from there, the Lord Jesus Christ, who, by the power that enables him to bring everything under his control, will transform our lowly bodies so that they will be like his glorious body." NIV translation
Paul reminds us that the earth that we see today is not our home, if we are in Christ. We are waiting for Him to renew all things and that includes our bodies. We will not reach the goal of being Christlike in all things until He returns and changes us. But, that does not mean that we conform to the ways of this world. Those that are at home (citizens) of this world will stay here until the day of judgment while those that are citizens of heaven through Jesus Christ will go to be with Him.