There is neither Jew nor Greek, there is neither slave nor free, there is neither male nor female; for you are all one in Christ Jesus. (Gal. 3:28)In these days in which the news reminds us of humanity's many divides, Paul's words to the Christians at Galatia remain a balm. In his new faith, there were meant to be no distinguishing nations, no statuses, no genders: all should be seen as one within the embrace of Messiah Jesus. Today we should include even those who don't want the embrace.
The 14 New Testament letters attributed to Paul and addressed to various Christian communities introduce many issues that—then and now—seem to be of near-obsessive interest to some Christians, but simply do not appear in the gospels. For example, Jesus never spoke about homosexuality nor abortion nor birth control, even although all three were practiced in the ancient world.
Other differences with Paul are significant.
Jesus leaves little or no room for divorce (Mark 10:2-12, Matthew 5:31-32 and 19:3-12 and Luke 16:18), Paul slips in a whopper of an exception—if your spouse is an unbeliever (1 Corinthians 7:10-15).
Except in the gospel of John, Jesus is ambiguous and almost reticent about who he is and whether he is divine; Paul is all over Jesus as the Son of God.
Tiptoeing around Jewish observances, Jesus technically breaks the Sabbath only to be charitable and the last supper suggests Jesus observed and knew Jewish dietary and ritual blessings. Paul scratches out circumcision and the whole of dietary law for Gentiles who convert to the Christian faith.
Of course, Paul was writing to mostly Greek Gentiles. To his intended readers Jewish law was entirely foreign and at odds with their customs. For example, anyone who has read Plato's Symposium knows perfectly well that the learned of Athens regarded pederasty and adult same-sex activity with benign eyes (even if Plato himself, along with Paul, did not). Hence the fulminations in Paul's letters against a variety of sexual activities.
Jesus was vague as to how he thought his followers should organize. The one exception is the saying about Peter (Matthew 18:16), which can be read a variety of ways in addition to the traditional view that the passage institutes the papacy. My own non-scholarly view is that the passage is a retrojection of the state of Christian communities at the time the gospel was written, in particular the city of Antioch, where Peter was bishop for a time.
Paul challenged Peter on the Gentiles question (which hid a dual "political" issue within the movement) and seems eager to direct the new communities. However, the three letters attributed to him that touch extensively on church order—I and II Timothy and Titus—are by scholarly consensus the work of Pauline followers rather than penned by the man from Tarsus himself.
In sum, Paul, a pivotal figure in transforming the Jewish Yeshua movement into what we know as Christianity, introduced some ideas later deemed foundational to the faith that cannot be directly traced to recognized teaching of Jesus.