Genesis 12:7

And the Lord appeared unto Abram
Perhaps in an human form, and so it was the Son of God; for whenever there was any visible appearance of a divine Person, under the former dispensation, it seems to be always of the essential Word, that was to be incarnate, and who spake with an articulate voice:

and said, unto thy seed will I give this land;
the whole of it inhabited by Canaanites and others; and it was for this end chiefly that Abram was called out of Chaldea into Canaan, to be shown the land, and have the grant of it for his posterity:

and there builded an altar unto the Lord, who appeared unto him:
by way of gratitude and thankfulness for his kind and gracious appearance, and for the gift of the land of Canaan to his offspring; for on this altar he no doubt offered sacrifice in a way of thanksgiving, as Noah did when he came out of the ark.