For my daily quiet time right now, I'm reading through the Book of Judith. Judith 5:6-9 caught my eye:
"[The Israelites] are descended from the Chaldeans. At one time they lived in Mesopotamia, because they did not wish to follow the gods of their ancestors who were in Chaldea. Since they had abandoned the ways of their ancestors, and worshiped the God of heaven, the God they had come to know, their ancestors drove them out from the presence of their gods. So they fled to Mesopotamia, and lived there for a long time. Then their God commanded them to leave the place where they were living and go to the land of Canaan" (NRSV).
According to this passage, Abraham abandoned idolatry and converted to the one true God in Chaldea. Then, he moved to Mesopotamia, where God commanded him to go to Canaan.
There are biblical traditions that coincide with this picture, as well as traditions that seem to contradict it.
What coincides with it is Genesis 11:31-12:5. In Genesis 11, Terah and his family leave Ur of the Chaldeans to go to the land of Canaan, but they stop in Haran (in Mesopotamia) and stay there. Then, God calls Abraham and tells him to go to Canaan. And so this passage presents God calling Abraham out of Mesopotamia, not Chaldea.
What seems to contradict Judith's picture, however, are the passages in which God brings Abraham out of Ur of the Chaldeans, not Haran, Mesopotamia (Genesis 15:7; Nehemiah 9:7). Many try to reconcile these passages by saying that God originally called Terah to go to Canaan, since we see in Genesis 11:31 that Terah was heading there (for some reason). But, according to these preachers, Terah neglected God's call on his life, leaving the torch to his son, Abraham.
Joshua 24:2-3 also appears to contradict Judith's scenario:
"And Joshua said to all the people, 'Thus says the LORD, the God of Israel: Long ago your ancestors--Terah and his sons Abraham and Nahor--lived beyond the Euphrates and served other gods. Then I took your father Abraham from beyond the River and led him through all the land of Canaan and made his offspring many. I gave him Isaac[.]"
According to the Book of Judith, Abraham converted to the worship of the one true God in Chaldea, so he wasn't an idolater when he moved to Mesopotamia. But Joshua 24 says that he served other gods "beyond the Euphrates," which refers to Haran in Mesopotamia. Haran is on the other side of the Euphrates, whereas Ur is right in front of the river. Joshua 24 seems to assert that Abraham converted to the one true God when God called him in Mesopotamia.
That inspires me to ask about other traditions in the history of interpretation. When do they say that Abraham left idolatry to serve the one true God? I've not surveyed the entire history of biblical interpretation, but there are sources that come to my mind: Josephus and rabbinic literature. For the latter, I'll take a look at the Jewish Encyclopedia and see if it references other sources as well.
According to Josephus, Terah left Chaldea because his son Haran died there (AJ 1:152). Terah obviously wanted a change of pace, since Chaldea made him sad. But Antiquities 1:157 suggests that Abraham was persecuted for his belief in God in Chaldea: "For which doctrines, when the Chaldeans, and other people of Mesopotamia raised a tumult against him, he thought fit to leave that country; and at the command, and by the assistance of God, he came and lived in the land of Canaan." Josephus seems to assume that Babylon is part of Mesopotamia, which may be true, even though Ur technically isn't between the Tigris and the Euphrates rivers (at least not according to maps I've seen).
According to the Jewish Encyclopedia (see ABRAHAM), other Jewish sources line up as follows:
Philo presents Abraham revolting against Chaldean astrology ("On Abraham" 17).
In Jubilees 11-12, Abraham destroys his father's idols and learns of the one true God in Chaldea. Terah then moves with his family to Haran, which is where Abraham is called by God.
According to Genesis Rabbah 38:13's interpretation of Genesis 11:28, Abraham destroyed his father's idols and was persecuted by Nimrod in Chaldea.
And so Jewish sources present Abraham converting to God in Chaldea, not Haran. This is probably because Babylon epitomized for them an anti-God system, since it destroyed the first temple and sent the Jews into exile.