On the other hand, mainstream Christian ideas about human nature, about sin, about the messiah, and about God's will (not to mention the nature of God), are for the most part based on Paul's writings and all represent departures from Jewish thought on those topics. It was primarily Paul who pushed for Christians to abandon kosher laws and circumcision, two major hurdles he faced in convincing non-Jews to join the new religion. More than anyone else described in the New Testament, Paul was responsible for Christianity becoming a distinct religion rather than just another Jewish sect.
Christianity is a multifaceted historical phenomenon, a cultural tradition of which Jesus and even the text of the Bible is a small part.
Saying Christianity is about what Jesus said and did makes as much sense as saying that the US is about the Declaration of Independence and the Constitution. In both cases, it's not nothing, but neither is it the whole story, or even 10% of the story.
You're free to decide what Christianity means to you, but not to prevent others from doing the same for themselves. After all, others doing it for themselves is precisely why "Christianity" is a family of seemingly countless denominations and divisions within. It's not a single monolithic thing, or even close to it.
Agreed, Jesus Christ’s teachings play a minuscule part, especially in modern American Christianity.
Do you think it's any less than among Catholics in Italy? Or Eastern Orthodox adherents in Russia?
So you may think people in the US merely profess being Christian according to your personal definition of whatever you think a Christian is or ought to be.
But I can guarantee you the US is an extremely religious country and the different denominations are quite sincere in their beliefs.
You don't have a monopoly on judging what "actual" Christianity is. In fact, your attitude is an extremely paternalistic, and frankly offensively, literal "holier than thou" position.