King Herod was the son of Antipater from Idumea. His mother came from Petra, the capital of the Nabataean kingdom.
Idumea was the area south of Judah where the Edomites lived. At the end of the second century BC John Hyrcanus subjected the Edomites and demanded that they must keep the Jewish laws. Therefore the Idumeans were circumcised and became part of the Jewish nation.
Antipater became governor of Idumea and also gained influence over Judea and Galilee. After his death, Herod conquered Jerusalem, with the help of the Romans. In order to pacify the Jews and strengthen his position as king, he married the Jewish Mariamne. He claimed to be a Jew, but didn’t keep the Jewish laws.