Maimonides (MT, 'abodat kokhabim, 1:3) explains that since his early childhood and while everyone else was adoring human-looking idols, Abraham began to think out of the box. His questions were practical questions: Everything moves --the sun, the moon, the stars-- but Who is moving everything? Most people, then and today, live their lives without this type of inquiries. They just follow and do what everyone else follows and does. But Abraham was different. He had reasonable doubts out of his own intellectual quest. When he was forty years old he arrived to the conclusion that there is a God, a Creator and ruler (manhig) of the universe. Abraham also realized that God is one. And furthermore, he was also able to conceive an idea unthinkable in those days for everyone else: that God is invisible! He had not teacher, no mentor and no partners in these ideas. He was surrounded by idol worshipers, including his own family. No one was curious like him to search the ultimate cause of the world's constant dynamics, etc.
There was one more extraordinary thing about Abraham: he was not satisfied with knowing the truth. He had to get the word out! Maimonides explains that Abraham held public debates with the people of Ur Kasdim. He advocated for his revolutionary ideas in front of his neighbors and family members. And became the first iconoclast (=destroyer of images and idols). The Mesopotamian king, Nimrod, viewed Abraham as a dangerous instigator. Let us not forget that the kings considered themselves gods. The new ideas of Abraham were a threat to Nimrod's divine status and to his unquestionable authority. Nimrod condemned Abraham to death. Maimonides mentions that miraculously Abraham escaped Nimrod's sentence and run away to Haran. Reaching eventually the land of Kena'an (Israel) where he dedicated the rest of his life to publicize his ideas about God.