Interpreters universally consider Genesis 12:1-3 to provide the key for the rest of the book. The verses constitute a fulcrum text, thoroughly theological in focus. They provide a link to what precedes ("all the families of the earth") and what follows (especially the promises). God is the speaker throughout and is the subject of the promises given. The promises focus on nationhood (which implies descendants), renown, and blessing for Abraham's family and for others through them (though Abraham will also be blessed by others, 12:3).
These promises are somewhat general, yet the emphasis on greatness entails a level of particularity that can be discerned by careful readers. The promises are brought into play in the following narratives again and again in various formulations (for example, 28:13-15), with descendants and land being made more specific (for example, 15:18-21).
The final phrase ("all the families of the earth") presents the ultimate objective of all the prior divine promises, namely, blessing for all the families of the earth. This family will be God's primary agent in and through which God's cosmic purposes are to be realized. God's election of Abraham is thus said to serve mission (in the broadest sense of the term). The call of Abraham may thus be said to constitute God's response to the dilemma created by human sin that has become so pervasive among all the families of the earth.
12Now the Lord said to Abram, Go from your country and your kindred and your fathers house to the land that I will show you. 2I will make of you a great nation, and I will bless you, and make your name great, so that you will be a blessing. 3I will bless those who bless you, and the one who curses you I will curse; and in you all the families of the earth shall be blessed.*
New Revised Standard Version Bible: Anglicized Edition, copyright © 1989, 1995 National Council of the Churches of Christ in the United States of America. Used by permission. All rights reserved worldwide. http://nrsvbibles.org