This week's parasha starts with Hashem speaking with Avra(ha)m and telling him to leave his home and move ... somewhere. There is no introduction as to why Hashem was telling him to do this or speaking to him at all. Our introduction to Avraham at the end of last week's parasha tells only of his family relationships, not anything he did that would warrant the prominent role in world history he was slated for. In general, not more than a sliver of the greatness of the forefathers can be seen through the narrative, but there are some basic features that explain their roles. Yitzchak is the righteous son of his two righteous parents, who was predestined to continue the chain. Yaakov was the pure tent dweller, about whom his mother prophesied that he would be the father of the chosen nation. Even Noach is introduced as a pure tzaddik who found favor in Hashem's eyes before being spoken to. How does Avraham just "explode onto the screen" of world history?
The Maharal (Netzach Yisrael 11) gives the following answer. Avraham was not chosen as an individual just as some sort of reward for what he did. Rather Hashem chose the nation which would descend from Avraham as His nation. Granted, this is in no small part to the credit of Avraham, and we are to this day dependent on the merit of the patriarchs' and matriarchs' good deeds. However, the Torah's silence on Avraham's pre-choice accomplishments is to stress the fact that this was not the main reason for his selection.
The Kuzari (95) discusses the preoccupation with the lineage from Adam to Avraham, through the tzaddikim Noach and Shem. Avraham, he posits, had the correct pedigree to succeed in his spiritual task. The link on the chain directly before Avraham was his father, Terach. Most of what we know about him is from Chazal, who present him as an idol worshipper who apparently partially improved his spiritual performance. Avraham was willing to have his holy son marry only a member of his father's house, even if her nuclear family consisted of idol worshippers. Apparently, Terach had elements of greatness in him or at least pass through him, although they did not find full expression until Avraham.
Going back to the Torah, we see that Avraham's great sojourn to the Land of Cana'an was started by Terach (Bereishit 11:31), who is described as taking Avraham and others with him. Terach was unable to complete the mission, but he blazed the trail, to a certain degree, of his illustrious son. Perhaps part of the reason that Avraham was chosen is that he was able to connect to his father and take the kernel of greatness within his father and perfect it.
Our generation has seen many ba'alei teshuva who rejected the secular lifestyles of their parents and returned to their roots. However, as descendants of Avraham, it is likely in many cases that they did not start from scratch, but tapped into the positive of their direct mothers and fathers as well as their forefathers. They have been able to actualize spiritual journeys that their parents began, and finish them in the Holy Land, whether literally or figuratively.