Paul makes his case in Jerusalem in the beginning of Chapter 22. He starts out by getting their attention. He is not getting attention just to glorify himself; his whole drive is to spread the good news about Jesus. He knows he needs to gain credibility for them to come to know Jesus, so he speaks in Hebrew (v2). My guess would be that he does not have a bad accent. Even when Peter speaks, the others comment on his accent right before he denies Jesus (Mt 26:73). My bet is that since Paul grew up in Jerusalem (v3), he was able speak well. He also drops the name Gamaliel. Gamaliel was a renowned teacher and was highly respected by most Jews at the time.  

Paul is a very interesting man. He is born in Tarsus, so he has Greek origin, but he is a Jew. Not just a Jew, but one of the strictest observers. He was known to follow the law to the letter. He killed many Christians for following their faith in Jesus. He even talks about how he himself was there when Stephen was killed, and how he gave personal approval for his death (v20). He was also a Roman citizen (v26). This he used to bring the faith to Rome. What a strange combination. He is considered the epitome of what it means to be Jewish – he has full rights from Rome, and is versed in the classics of Greece.
Paul admits his wrongdoing to the Christians. He gives his witness story of how he came to know Jesus and of being forgiven by Him. Paul is ashamed of his previous behavior, but he uses it to glorify Jesus. In this talk, he particularly glorifies Jesus’ mercy. He talks about how God was so good to him, even when he did not know it. Has God been good to you? Do you ever tell anybody those times when Jesus was there for you? Paul talks about something that was embarrassing from the past, but our stories that we tell do not have to be. They are simply stories of God’s goodness.
After receiving his sight, Paul gets up and was baptized (Acts 22:16). In the same sentence he describes what this baptism does for him, “…and your sins washed away…” Belief that baptism washes away our sins is well established here. Acts 2:38 is even more specific and clear. Peter said to them, “Repent and be baptized, every one of you, in the name of Jesus Christ for the forgiveness of your sins; and you will receive the gift of the Holy Spirit.” It even adds that we receive the Holy Spirit by being baptized.
Catholic understanding of this is that in order to receive the Holy Spirit, we must be free from sin. Nothing impure can touch the Ark of the Covenant (2 Samuel 6:1-7). Moreover, nobody could even see God and live (Ex 33:20). This is because fallen humanity is not free from sin. We have already sinned and we are also finite and dependent. We might think we are independent, but in reality, we are not. We need others to grow food, supply needs, and still need someone to teach us and supply work for us, and the list could go on. In the case of seeing God or having Him live in us, we need Him to make us free from sin. The good news is that God is gracious enough to do just that – make us free from sin so that He may live in us. God does most of the work.
After Paul’s witness, the Jews still have him arrested. This is where he claims his Roman citizenship. It almost makes him an elite. The guards are now afraid of the fact that they bound him, let alone would have treated him badly. So an appeal to the Sanhedrin is made to find out more about what the accusations were.