In the days of Avraham Avinu, most people worshiped idols. The Midrash relates that Avraham's father, Terach owned an idol shop. Avraham realized that these idols, which were man made, were lifeless and could not possibly have any domination over the earth. There had to be a superior force capable of creating the world and controlling the forces of nature. One day Avraham looked at the sky and saw the brightness and beauty of the sun. "This must certainly be the ruler of the world," and he started to worship it. But at night the moon appeared along with the stars. Avraham now thought that it was the moon and stars that were the rulers of the world, and he worshipped them. Once he did this, he watched as the moon set and the sun rose. Avraham then realized that it was not the sun nor the moon or stars that ruled, but a power even greater. This was when Avraham worshipped Hashem.
Avraham was the first individual to realize on his own that Hashem was the one, true God. He then spread the belief in Hashem to all future generations. It is said that even a child can comprehend the existence of Hashem. A non-Jew once asked a child how he knew that Hashem existed. The boy responded that he lived by the seashore. When he would see footprints in the sand, he would reason that a person must have gone by. Similarly, when he saw the sun, moon and stars, he could reason that Hashem must have created them. Hashem is truly Master of the universe.