Philippians 3:3 For we are the real circumcision, who worship by the Spirit of God and glory in Christ Jesus and put no confidence in the flesh- 4 though I myself have reason for confidence in the flesh also. If anyone else thinks he has reason for confidence in the flesh, I have more: 5 circumcised on the eighth day, of the people 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 righteousness, under the law blameless. 7 But whatever gain I had, I counted as loss for the sake of Christ. 8 Indeed, I count everything as loss
because of the surpassing worth of knowing Christ Jesus my Lord. For his sake I have suffered the loss of all things and count them as rubbish, in order that I may gain Christ 9 and be found in him, not having a righteousness of my own that comes from the law, but that which comes through faith in Christ, the righteousness from God that depends on faith- 10 that I may know him and the power of his resurrection, and may share his sufferings, becoming like him in his death, 11 that by any means possible I may attain the resurrection from the dead.
People were teaching false things concerning salvation from sin to the Philippian Christians and claiming it was the truth. Paul gave us an example to follow. He refuted error with Truth. Most of us don't have time to learn all the false teaching, but we do have time to learn the Truth by daily reading or listening to the Bible and learning it.
Paul refutes the false teaching (that we need to be circumcised to be saved from our sin) with the Truth. He says of Christians, "We are the circumcision, who worship by the Spirit of God and glory in Christ Jesus and put no confidence in the flesh." The false teachers instruct people that they are saved from sin by what they do. Paul taught the people that they are saved by what they believe Christ did. Jesus Christ died on the cross as payment for our sins, rose again from the dead as proof He paid for our sins in full, and ascended to His throne to rule as Lord over all.
Paul answers the question, "But what about the people who live a good life?", with his testimony. Paul testifies that he lived life better than all others, but it wasn't good enough to gain freedom from the penalty of sin, which is eternal torment. Paul stated that he willingly gave up credit for all he had ever done when he came to know Christ and believed that what He (Jesus) did saved him from the penalty of sin.
Are you trusting your salvation from the penalty of sin comes from what you do or from what you believe Jesus did for you? Our salvation comes when we believe Jesus Christ died for our sins on the cross, rose again from the dead on the third day and is Lord of all. Jesus calls us to repent of our sins and believe in Him.