Everyone in Mesopotamia worshipped the stars. The sun, the moon, the constellations—they were the gods of Chaldea, and no one questioned it. No one except Abraham.
According to Josephus in his Antiquities, Abraham arrived at monotheism not through a vision or a voice from heaven, but through pure reason. He looked up at the sky and noticed something that bothered him. The heavenly bodies were irregular. The sun set when it shouldn't. The moon waned unpredictably. The stars drifted. If these celestial objects were truly gods, Abraham argued, they would at least be able to control their own movements. They couldn't. Which meant they were servants, not masters.
This was a radical idea—so radical it nearly got him killed. The Chaldeans turned against him. The people of Mesopotamia raised what Josephus calls a "tumult," furious that this man would dare challenge the gods they had worshipped for generations. Abraham didn't back down from his reasoning, but he did leave the country, traveling to the land of Canaan at God's command (Genesis 12:1).
Once there, he built an altar and offered a sacrifice—the first act of worship by a man who had reasoned his way to the one God.
Josephus wasn't the only ancient writer who remembered Abraham's fame. He cites Berosus, the Babylonian historian, who described a righteous man "skilled in the celestial science" living in the tenth generation after the Flood. And Nicolaus of Damascus recorded that Abraham once ruled in Damascus as a foreign king who came from the land of the Chaldeans—and that a village there still bore his name centuries later.