Jesus was a cult leader in Roman-era Judea. He appears to have absorbed some of the followers of John the Baptist, a previous cult leader (though this may have happened later). And then another cult leader named Paul absorbed some of Jesus’ followers. Jesus appeared to be concerned about reforming Jewish practices, and his cult seems to have syncretistically combined elements of Judaism and Hellenic mystery religions (baptism has roots in mysteries). He appears to have both pissed off the Jewish establishment and the Roman government.
We don’t know much about what Jesus really said or thought because the earliest Christian writings we have are from Paul, who never met Jesus and who re-appropriated Jesus’s historical person into an Osiris-like salvific figure. The gospels were written several decades after Jesus’ death. But parts of the gospels were probably copied from earlier sources, some of which may have reflected Jesus’ actual sayings. If we examine these closely, a wide variety of these “sources” portray Jesus as being against divorce. So that’s the thing I’m most sure about Jesus: he didn’t like divorce.
To all the people who say that Jesus was a myth or didn’t exist: Fine. Explain where the pre-Pauline cult of his followers came from. If Paul and Peter invented Jesus out of thin air, why was there already a well-formed cult of Jesus-followers by 50AD, when Paul wrote his first letters?