The term "Jew" comes from the fact that 11 of the 12 tribes of Israel were conquered by the Babylonian King and shipped elsewhere into his empire. The only tribe not conquered was the tribe of Judah, whose land was called Judea. Thus, the Israelites today claim descent from that tribe. Hence, they are Jewish--pronounced differently elsewhere (English muddles words). I don't know how they can be "ish". One is either a Jew or they aren't.
The Judean man from Galilee who was crucified when he came to Jerusalem in the 30s AD was named Yeshua (Aramaic for Joshua). Saul of Tarsus, when preaching Yeshua's message, decided to expand the religion to the Greeks in Corinth, Ephesus, etc. This angered the Jewish followers, because they did not want their religion shared with people who were not circumcised. In order to sell the religion to the Greeks, Saul had to alter the religion and make it into a "Greek" one. So, Saul changed his own name to the Greek name of "Paul". He gave Yeshua the Greek-sounding name of "Jesus". He gave him the Greek title of "Christ". He created some similarities with Greek religion, i.e. Hercules, humanlike son of Zeus = Jesus, humanlike son of the Hebrew God. He altered Jesus' stories and made it so that the Greeks did not have to have dietary restrictions or mutilate their genitals to join. He even added Persian concepts of good/evil and heaven/hell. I suspect money was behind this desire to evangelize to the Greeks, but I wouldn't know. In the 60s AD, Emperor Nero began a mass-persecution of Jews (including those who followed Yeshua's message). The great Jewish revolt in Jerusalem that followed led to the Roman-Jewish War which led to the end of Jewish sovereignty, the collapse of the royal family in Rome and the rise of the Roman generals who would take the throne. Long story short, the Jewish followers of Yeshua were exterminated by the Romans. Only the Greek followers of Christ remained throughout the empire. After the war ended in 69 AD, the four Gospels began to appear and circulate. Thus, Christianity was born.
I should mention that the J sound was not and is not used throughout most of Europe. Julius Caesar was really named Iulius Caesar. It was the Norsemen I think in the 900s and 1000s who brought the J sound and added it to English words. So, in English, Yeshua became Joshua. "Yesus" became "Jesus". That was a big answer for a big question.