They have abandoned their faith. Since the Jewish faith is at the core of Jewish culture and traditions, I wonder what culture and traditions they are preserving? Bagels for brunch, Woody Allen movies and the New York Times?
Find out what "Jesus" did and then you will understand what they do....But I am sure you have no clue on how Jesus practiced his Judaism...
So you don't feel it is arrogant to declare that those Jews who believe the Messiah has already come are no longer Jewish?
I know of many observant Jews who also believe that Jesus is the Messiah. They are no less Jewish because of that belief.