I think it's easy to over-assume the importance of St. Paul. That isn't to deny his very real importance, only that we end up looking back to the apostolic era largely through Paul because Paul's writings form the largest bulk of what we have, and the only narrative in the NT apart from the Gospels--the Acts of the Apostles--is 2/3 about Paul.
But Paul wasn't the only one doing what he was doing, he frequently mentions the many people who were engaged in the same work he was doing--Apollos, Barnabas, Silas, and Peter etc. There is a tendency to perhaps imagine that there were The Twelve, and then there was Paul, with the Twelve doing their thing apparently without much noise after Paul shows up, and then there's superstar Paul. But Paul was just another apostle, in fact he refers to himself as the least of the apostles; keeping in mind that aside from the Twelve there were a number of apostles, again, Apollos, Barnabas, Silas, etc.
Paul was instrumental in his work with the Gentiles, but he wasn't an independent agent. He acted in accord with the rest of the Church; in the Acts we can recall how when Paul and Barnabas were preaching in Antioch they traveled to Jerusalem to meet with the Jerusalem leadership which, ultimately, gave them their blessing.
The hard part about the question is that historical what-ifs are largely speculative, but I think we err in assuming that a Christian history without Paul would somehow result in a radically different sort of Christianity--as though Paul was the only one reaching out to the Gentiles, Paul was part of the ministry to the Gentiles, but was by no means the only one. We can also talk about the importance of St. Paul's theology, but even though we talk about Pauline theology we might miss something if we think in terms as though it were uniquely his. There have been a number of profoundly important figures in the history of Christian theology--the 4th century alone produced some of the most important theological powerhouses in all of Church history: Athanasius, Hillary of Poitiers, Basil the Great and the two Gregorys, Augustine of Hippo--but if we ask the question, what would Christianity look like without Athanasius? What would Christianity look like without Gregory Nazianzus? No single person holds all the cards. The theological history of the Church is built on the shoulders of giants confessing a faith that came before them and which is much bigger than they are, bigger than any of us.
Without Paul perhaps we'd be more familiar with the work of St. Apollos, or the ministry of St. Barnabas, perhaps we'd be reading from the letters of St. Peter to the Corinthians, Ephesians, etc. We really don't know, but what we can say is that as important and instrumental the holy apostle Paul was, he wasn't a one man army, he was a piece of a much larger structure. Christ's holy catholic Church.
-CryptoLutheran
