In Luke 9:6, we read that the 12 apostles “went from village to village proclaiming the good news and curing diseases everywhere.” What was that “good news”? The Gospels were not yet written. Similarly, what did St. Paul preach?
The good news is the Gospel of Jesus Christ. The apostles had already lived it partially before Jesus first sent them out to preach it. They still had much to learn.
Euangelion is Greek for “good” (eu) and “announcement” or “news” (angelion). Latin renders this Greek word as evangelium, from which we get evangelist, evangelical, and related words.
The good news of Jesus was first lived, then spoken, and finally written. The Gospel of Mark, the first one to be completed, reached the form in which we know it about 40 years after Jesus’ death and resurrection. In the meantime, the good news lived through zealous preaching and generous works of mercy by all Christians.
Acts of the Apostles and Paul’s letters give us a good idea of what Paul preached. Your question about the content of his preaching indirectly affirms the role of the Church in our receiving the good news: it comes to us through a living, breathing faith community. Paul learned about it several years before he set out on his first missionary journey. The Christians in Damascus, Jerusalem, and eventually Antioch were his first teachers.
The earliest chapters of Acts record several sermons from Peter. Many Scripture scholars believe that the Second Letter of Peter is basically a homily explaining the Sacrament of Baptism.