Paul follows the typical style of ancient Greek letters. He starts with a brief introduction about himself, then offers greetings. However, Paul expands this basic pattern in a distinctly Christian way.
First, Paul refers to his divine calling as an apostle rather than just stating his name. This shows that his authority comes from God, not from himself.
Second, Paul describes the people he writes to as "God's holy people" and as "faithful followers of Christ." This reminds them of their special identity and relationship with God.