Ramban, on Gen 12:2

“And I will make of thee a great nation, and I will bless thee, and magnify thy name, and thou shalt be blessed.”

“AND BE THOU A BLESSING. You will be the blessing by whom people will be blessed, saying, “G-d make thee as Abraham.” To this He added that all families of the earth will cite him in blessing, not just the people of his country alone. It may be that the expression, And in thee shall all the families of the earth be blessed, means that they will all be blessed on his account. Now this portion of Scripture is not completely elucidated. What reason was there that the Holy One, blessed be He, should say to Abraham, “Leave your country, and I will do you good in a completely unprecedented measure,” without first stating that Abraham worshipped G-d or that he was a righteous man, [and] perfect? Or it should state as a reason for his leaving the country that the very journey to another land constituted an act of seeking the nearness of G-d. The custom of Scripture is to state, “Walk before Me, and hearken to My voice, and I will do good unto you,” as is the case with David and Solomon,, 3:13-14. as well as throughout the Torah: If ye walk in My statute; And it shall come to pass, if thou shalt hearken diligently unto the voice of the Eternal thy G-d. And in the case of Isaac, it says, For My servant Abraham’s sake. But there is no reason for G-d to promise [Abraham a reward merely] for his leaving the country. However, the reason [for G-d’s promising Abraham this reward] is that the people of Ur of the Chaldees did him much evil on account of his belief in the Holy One, blessed be He, and he fled from them to go to the land of Canaan, tarrying for a time at Haran, whereupon the Eternal told him to leave these places as well and to fulfill his original intention that his worship be dedicated to Him alone and that he call upon people [for the worhip of] the Name of the Eternal in the Chosen Land. There He would make his name great, and these nations would bless themselves by him, not as they treated him in Ur of the Chaldees, where they abused and cursed him, put him in prison or in the fiery furnace. He further told Abraham that He will bless those who bless him, and if some individual will curse him, he will be cursed in turn. This then is the meaning of this portion of Scripture. The Torah, however, did not want to deal at length with the opinions of idol worshippers and explain the matter between him and the Chaldeans in the subject of faith, just as it dealt briefly with the matter of the generation of Enosh, Note 280. and their thesis concerning the idol-worship which they instituted.”

Imported from an open dataset — not yet checked against the printed edition.