Jesus was younger than Abraham, so when Jesus claimed to exist before Abraham their father was born, Jews thought he has a demon and he is a demon.  They were confident that he is a liar.  Jews, did not want to admit that as Abraham was their greatest father.  They did not know Jesus or God himself.

that would mean that he was GOD and to them that meant that he blasphemed, they did not understand or were convinced that JESUS was part of the HOLY TRINITY