NOTES for Быт 12:3
God's promise is given to Abram (it is later that he will be called Abraham), but it spreads out (quite as this entire testament) to all his descendants. And we know indeed that God never violate His promises, He fulfills them sooner or later independently of the fact, people respect the covenant with him or not, because God is faithful.
Let us think now what it means for us today. God promises to bless those who bless the descendants of Abram and curst those who curse them. And no behavior - worthy or unworthy- of the descendants of Abram throughout the history of humanity changes nothing in God's promise. Yes, all nations of the world receive blessing through one of the descendants of Abram, Jesus Christ, and by the faith in Him we are all blessed, but even this does not abolish the curse promised by God, if we don’t believe in His words said to Abram.