Jamieson, Fausset and Brown on Genesis 12:7

7. Unto thy seed will I give this land--God was dealing with Abram not in his private and personal capacity merely, but with a view to high and important interests in future ages. That land his posterity was for centuries to inhabit as a peculiar people; the seeds of divine knowledge were to be sown there for the benefit of all mankind; and considered in its geographical situation, it was chosen in divine wisdom as the fittest of all lands to serve as the cradle of a divine revelation designed for the whole world.

and there builded he an altar unto the Lord--By this solemn act of devotion Abram made an open profession of his religion, established the worship of the true God, and declared his faith in the promise.