Answer:
The whole point of the Jewish religion is that there is only one G-d. During the days of the founding father Avraham, everyone else around him worshipped idols. He knew that the manmade stone idols were not G-d, as they couldn't walk, hear, or talk. Avraham realized that there could only be one G-d when he started to observe what the others considered to be the real Gods. First he observed the sun (which some considered god) but then at the end of the day, it went down. Avraham knew that if the sun was god it wouldn't just go down for the moon to come up. And the same thing happened with the moon, it went down in the morning so that the sun could come up. Avraham then realized that the real god couldn't be the sun or the moon. In the end, Avraham realized that the one and only real god was not a physical thing.
This knowingness about Gods true form and being is the core of Judism. Jews believe that there is only one God and that is who they serve, not a made up stone idol or something else that is not real.
