He warns about the Judaizers who teach that circumcision is a must for one to be saved. He called them dogs, evil doers and mutilators of the flesh. The teaching that circumcision is necessary for salvation is not of God and Paul stands firmly against them. He then declares he and the followers of Christ are truly circumcised in a way that brings the right relationship with Christ. The old testament circumcision of the flesh is a matter of human hands and of the flesh while his circumcision is a circumcision of the heart creating the saving relationship with Christ. He declares that if there was anyone who could have confidence in living rightly in the flesh according to the customs and teachings of the law, it was him.
Paul's efforts were widely known as being done well beyond the abilities of most men. He was zealous in his expression of his Old Testament faith and executed it's teachings to the letter. He lists his accomplishments as being circumcised at 8 days old as per the law. He was a pure blooded citizen of Israel and of the tribe of Benjamin, taking his roots back to the chosen people of God, making him a "real" Hebrew. He was a Pharisee and as such followed the law with strict attention to it's requirements. He obeyed the law without flaw and even persecuted the Christians.
All the things he did and had just listed were things he once thought were the right way. Now having found Christ, he counts all his former accomplishments as junk. Knowing Christ is more valuable than anything he ever did or imagined. Learning Christ as Savior, he discarded all the things he thought right and true. His new desire is to be one with Christ, realizing that whatever he did thinking he could become righteous was wrong, for he now knows righteousness can only come through Jesus Christ. Paul is now willing to do anything and everything so he will experience resurrection from the dead by the mighty hand of Christ.
He wants the Philippians to make sure they don't misunderstand what he is saying. He clarifies that he is not perfect now, but that he is pushing forward doing his best with a heart full of desire to accomplish a right life. He presses onward to possess the perfection that comes through Christ not through his own effort. He has therefore walked away from the old teachings and holds on dearly to the teachings of Jesus. He likens life as to a race and his desire is to finish his race (or life) following Christ to receive his prize, eternal life. He then mentions that all who are mature should understand and agree on that thinking. Also, that all must hold on to the progress they have made and not ease up or slide backward.
Paul closes chapter 3 with some instructions. First, pattern your lives after his and learn from those who follow the way he lives. They are to use his life and teaching as the example because there are many who pretend to be followers of Christ but are just acting in order to accomplish evil plans. He warns not to follow them for their path will lead all who follow them, to destruction. The truth of what they desire can be spotted because they care only about things of the world and those are the things they pursue, not Christ.
He declares he and the followers of Christ are not like those evil doers. He states they are citizens of heaven, the home of Jesus Christ. While they live on earth, they are eager for Christ to return as the Savior. When He comes He promises to transform our weak, mortal bodies into glorious bodies like His. He will make this happen by using the same power with which He will bring all things under His control. We can rest in His promise and in His power for He offered Himself, took our shame and punishment, died on the cross, was buried and rose from the grave to sit in heaven at the right hand of the Father. Christ is trustworthy and will do just as He promised.
Now we wait for Him to come again as He promised. We have faith in what He said He would do because He fulfilled all His other promises. We can live, looking forward to the future with all confidence that He will again accomplish all He promises.