The call of Abraham in Genesis 12 is one of the most pivotal moments in all of Scripture. After the Table of Nations in Genesis 10-11 and the scattering at Babel, God narrows his redemptive focus to one man - not because Abraham was morally superior, but because God operates through particular election to bless the universal.
The covenant God makes with Abraham (Genesis 15, 17) is a binding legal commitment - God himself passes between the divided animals, taking on the covenant curse. In ancient Near Eastern treaty-making, both parties walked through. Here, only God walks. It is an unconditional promise, not a contract Abraham can break.
I will make of you a great nation, and I will bless you and make your name great. - Genesis 12:2 (ESV)