Plus looking at ALL applicable scriptures from both testaments. For instance what do you do with this statement:?
Phil 3.5 circumcised the eighth day, of the nation of Israel, of the tribe of Benjamin, a Hebrew of Hebrews; as to the Law, a Pharisee; 6 as to zeal, a persecutor of the church; as to the righteousness which is in the Law, found blameless.
Do you think Paul really counted himself as sinless (regarding the Law)? Paul said of himself
The saying is sure and worthy of full acceptance, that Christ Jesus came into the world to save sinners--of whom I am the foremost.
1 Timothy 1:15
When Paul speak of himself as blameless in reference to the Law he is likely referring to his assiduous observance of the rules about separation, cleanliness, and so forth. He was not referring to himself as sinless because he knew himself to be egregiously wicked because "I am the least of the apostles, unfit to be called an apostle, because I persecuted the church of God" 1 Corinthians 15:9 and Luke records
When they heard these things, they became enraged and ground their teeth at Stephen. But filled with the Holy Spirit, he gazed into heaven and saw the glory of God and Jesus standing at the right hand of God. 'Look,' he said, 'I see the heavens opened and the Son of Man standing at the right hand of God!' But they covered their ears, and with a loud shout all rushed together against him. Then they dragged him out of the city and began to stone him; and the witnesses laid their coats at the feet of a young man named Saul. While they were stoning Stephen, he prayed, 'Lord Jesus, receive my spirit.' Then he knelt down and cried out in a loud voice, 'Lord, do not hold this sin against them.' When he had said this, he died.
Acts 7:54-60
And Saul approved of their killing him. That day a severe persecution began against the church in Jerusalem, and all except the apostles were scattered throughout the countryside of Judea and Samaria. Devout men buried Stephen and made loud lamentation over him. But Saul was ravaging the church by entering house after house; dragging off both men and women, he committed them to prison.
Acts 8:1-3
Have you looked carefully at the passage you quoted? It says
Finally, my brothers and sisters, rejoice in the Lord. To write the same things to you is not troublesome to me, and for you it is a safeguard. Beware of the dogs, beware of the evil workers, beware of those who mutilate the flesh! For it is we who are the circumcision, who worship in the Spirit of God and boast in Christ Jesus and have no confidence in the flesh-- even though I, too, have reason for confidence in the flesh. If anyone else has reason to be confident in the flesh, I have more: circumcised on the eighth day, a member of the people of Israel, of the tribe of Benjamin, a Hebrew born of Hebrews; as to the law, a Pharisee; as to zeal, a persecutor of the church; as to righteousness under the law, blameless. Yet whatever gains I had, these I have come to regard as loss because of Christ. More than that, I regard everything as loss because of the surpassing value of knowing Christ Jesus my Lord. For his sake I have suffered the loss of all things, and I regard them as rubbish, in order that I may gain Christ and be found in him, not having a righteousness of my own that comes from the law, but one that comes through faith in Christ, the righteousness from God based on faith. I want to know Christ and the power of his resurrection and the sharing of his sufferings by becoming like him in his death, if somehow I may attain the resurrection from the dead.
Philippians 3:1-11
Paul is not praising confidence in the flesh he is condemning it both in himself and in others. The words you made bold in your quote are intended to be scorned as wicked fleshly confidence that will end in condemnation unless repented of.