This is a great question. As you can imagine, Paul, apart from Jesus, is the most important person in early Christianity. There are many reasons for this, but I will only mention three of them.
First, of the 27 New Testament books, 13 of them have been written by Paul. To be sure, some scholars dispute this number, but in the end all would agree that his writings have made an indelible mark on the early church. What makes this even more profound is that his theological formulations are some of the most penetrating in the New Testament, such as the book of Romans and Ephesians
Second, Paul was the first one to preach the gospel to the Gentiles, that is, non-Jews. This is perhaps the most important point. Through the efforts of Paul, the message of Christianity was now opened to the rest of the world. For this reason, he can be said to be the first missionary and universal in is vision.
Third, Paul's ministry was in cities. This point, too, must not be under emphasized. Paul selected the most influential centers as bases for his ministry. For example, Rome was the largest ancient Greco-Roman city, Ephesus was the third greatest city in terms of number and the capital in Asia Minor, and the rest of the cities he visited were significant in some way. For this reason, we can say that he started an urban movement and that he was far ahead of his times.
His legacy lives on even today.