The gospel is the message that Saul teaches and in Acts 9:22, Luke also gives us an insight into his method. Saul proves Jesus Christ to be the Messiah. Saul reasons with people. He proves from the Scriptures that Jesus is the Christ. He quotes from the Old Testament. He explains how the life of Jesus matches up to the prophecies, how it brings an end to the sacrifices. He uses his mind. He uses logic, his knowledge of Greek and Hebrew. In disputing with the Hellenists, there is discussion and debate.
The reasoning of Saul serves to affirm the importance of theological training. We need preachers to know the Bible well and interpret it in the same manner as the apostles. It also shows us that proclamation is central to evangelism. Washing cars, distributing food and water, teaching English, farming or another skill: there are lots of helpful and important things that can be done to serve a community. Diaconal work is good and appropriate; it provides opportunities to make new friends, but it is not evangelism per se. Evangelism is proclamation. It involves teaching and explaining truth about who Jesus is and what he has done.
22 But Saul increased all the more in strength, and confounded the Jews who lived in Damascus by proving that Jesus was the Christ.