This transition is one that has occupied my interest for many years (I’m actually writing a book about it!). If we look at the beginning of Parshat Lech Lecha (Bereshit 12:1), it seems that God told Avraham, out of the blue, to go to Eretz Canaan. But a few verses earlier, at the end of Parashat Noach (Bereshit 11:31), we see that Terach took Avraham, and the rest of his family, and left Ur for Canaan, without a divine command. They stopped in Haran, and settled there.
That brief story, being followed by the command, “Lech lecha”, leads to many questions, and so it’s understandable why the parshiot break there. Why did God choose Avraham? He hadn’t done anything noteworthy before God spoke to him. Was there no back story or was the story not told? Was Avraham’s journey to Eretz Canaan the continuation of Terach’s (and what were his motivations?) or was it independent and only based on God’s command?
There are many more questions about the specific language used, Avraham’s age, where God spoke to him, and who went with him on the trip. As my research has indicated, there’s too much to describe here.
Many answers have been given. Among the more famous are the midrash about Avraham and the fiery furnace, Rambam’s explanation of Avraham as a philosopher, and Maharal’s claim that Avraham was chosen for no reason at all. A more recent, and fascinating, answer was given by Rabbi David Fohrman, who points out that Avraham (and Nachor) take in the children of their dead brother Haran, and that act of kindness was unusual, and was the justification of God choosing Avraham.
However, none of these answers explain my most basic question: if answer X is correct, why didn’t the Torah just write it that way? My theory (and this is really while “standing on one leg”), is that the Torah was intentionally ambivalent about why Avraham was chosen. On the one hand, it was important to have a story (or hints to a story) that indicated that Avraham was the one who made the choice to follow God. This would set the pattern for the later Sinai covenant, where our special relationship with God was dependent on following His laws. On the other hand, the future nation would never be fully abandoned by God, so it couldn’t be dependent on a particular action, no matter how noble.
So since both of these contradicting ideas needed to coexist, they were told in two concurrent stories. And to avoid the dissonance that reading them together would cause, the sages divided them where they did – between Noach and Lech Lecha.