What is the fundamental difference between Judaism and Christianity

I will not unpack this too much, but I would say the fundamental difference is that although they believe in the same God, the Jews believe that the messiah is yet to come, whereas the Christians believe that the messiah has already come in the form of Jesus. The Jews do not accept Jesus as the messiah as they believe the messiah will save them physically, whereas Christians believe the messiah, Jesus, came to save them spiritually.