Lekh Lekha | Vayikra BeShem Hashem: Avraham's Altars

Upon his arrival in the Land, Avra(ha)m builds a mizbeach; he then builds another one where he sets up his camp and there, he "calls out Hashem's Name"; he sets up another mizbeachwhen he settles in Chevron and then, again, calls out in Hashem's name at Beer Sheva, without building a mizbeach. How are we to understand the purpose of these altars - which are inherently meant for offerings? We also note that each of these acts is followed by a challenge, a calamity or a threat - until he builds his final mizbeach, atop Har haMoriah, and he offers up an animal (in lieu of his son) as the denouement of the Akeida. We analyze these passages in their context and then propose an explanation for Avraham's activity, as well as proposing a reason for the Akeida itself.