Why was Paul so upset? We have to remember his background. Here were these Judaisers coming to the Galatian Christians telling them that Paul’s Gospel was insufficient. In Acts chapter 22 Paul gives his testimony in Jerusalem, "I am a Jew, born in Tarsus of Cilicia, but brought up in this city. Under Gamaliel I was thoroughly trained in the law of our fathers and was just as zealous for God as any of you are today. I persecuted the followers of this Way to their death, arresting both men and women and throwing them into prison, as also the high priest and all the Council can testify. I even obtained letters from them to their brothers in Damascus, and went there to bring these people as prisoners to Jerusalem to be punished. “
Again in the letter to the Philippian church (Philippians 4:6) he declares, “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; as for zeal, persecuting the church; as for legalistic righteousness, faultless.”
Paul was a Pharisee, a religious group of Jews who were renowned for their strict keeping of the Mosaic Law. If the Law of Moses could save anyone it could have saved Paul. But something happened to him. On his way to Damascus to seek out Christians this: (Acts 22 again)
"About noon as I came near Damascus, suddenly a bright light from heaven flashed around me. I fell to the ground and heard a voice say to me, 'Saul! Saul! Why do you persecute me?' 'Who are you, Lord?' I asked. 'I am Jesus of Nazareth, whom you are persecuting,' he replied. My companions saw the light, but they did not understand the voice of him who was speaking to me.”
"'What shall I do, Lord?' I asked. 'Get up,' the Lord said, 'and go into Damascus. There you will be told all that you have been assigned to do.'”
Paul was constantly talking about his encounter with Christ. We first read about it Acts chapter 9. In Acts 22 and later in Acts 26 before Agrippa he relates his testimony. In Galatians chapter 1 he tells the whole story in letter form, filling out some of the details of what happened next and in 1 Corinthians 15 he refers to the event obliquely ‘and last of all he appeared to me also as to one abnormally born.’ (v 7). For Paul, this one event changed his attitude to religion. It ceased to be a matter of keeping the rules but instead what mattered was his relationship with Jesus Christ.
As a hint on evangelism: we are all required to witness, but we are not all required to be theologians. What can you tell your friends about Jesus? Tell them about your encounter with Christ. Give them your testimony. After all it convinced you.