"And I will make you a great nation; I will bless you, and I will make your name great, and you will be a blessing." (12;2)
This is the first time that the torah records Hashem speaking to Avraham, and yet we read that Hashem promised him a lot of great things. The question arises: Why didn't the torah mention any previous discussions between Avraham and Hashem?
Similarly, Rashi (11;28) tells us that Avraham was miraculously saved from a fiery pit and that he had already caused a mass of people to convert (Rashi,12;5). However, if Avraham was so righteous then why didn't the torah mention him beforehand?
Rabbi Yochanan Zweig explains that Hashem spoke to Avraham because He had an ongoing love relationship with him-not because of his righteousness. Avraham had an eternal relationship with Hashem. Therefore, Hashem told him that He would make him eternal by making him into a great nation.
Rabbi Zweig explains that the torah only lists things which have some connection to Hashem's relationship with the nation of Israel. The torah doesn't write things in order to tell us how great certain people were. For this reason, Rabbi Zweig explains that the torah didn't explicitly mention that Avraham was miraculously saved from the fiery pit. Every single pasuk in the torah is written because it had an effect on the relationship between man and Hashem. As a result, none of the great actions Avraham did before this point were written in the torah (I.e. converting a huge mass of people).
Similarly, the torah says nothing about Yitzchak from the age of 122/123 until he died at 180. And why is that? Rabbi Zweig explains that the torah only writes incidents which changed, for the better or worse, our relationship with Hashem. Yitzchak failed to enhance his relationship for nearly the last 60 years of his life. Thus, Rabbi Zweig explains that there is no mention of him in the torah for that period of time.