When God said this...
Gen 12:3 KJV And I will bless them that bless thee, and curse him that curseth thee: and in thee shall all families of the earth be blessed.
What that means is this...
Gal 3:8-10 KJV And the scripture, foreseeing that God would justify the heathen through faith, preached before the gospel unto Abraham, saying, In thee shall all nations be blessed. (9) So then
they which be of faith are blessed with faithful Abraham. (10) For as many as are of the works of the law are under the curse: for it is written, Cursed is every one that continueth not in all things which are written in the book of the law to do them.
Israel under the law is not blessed but under a curse. When it said the blessing of Abraham it refers today to those who have faith in Jesus.
Everyone really needs to read that carefully.
And then...
Gen 17:8 KJV And
I will give unto thee, and to thy seed after thee, the land wherein thou art a stranger, all the land of Canaan, for an everlasting possession; and I will be their God.
Paul distinctly said this about that phrase:
Gal 3:16 KJV Now to Abraham and
his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ.
Gal 3:29 KJV And
if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise.