Genesis 12 records the call of Abram and the beginning of his establishment as the father of many nations. God promises, “to your offspring I will give this land,” and later says, “I will establish my covenant between me and you and your offspring after you throughout their generations for an everlasting covenant, to be God to you and to your offspring after you.”
Paul makes it clear in Galatians 3 that this promise is ultimately given to and fulfilled in Christ. “Now the promises were made to Abraham and to his offspring. It does not say, ‘And to offsprings,’ referring to many, but referring to one, ‘And to your offspring,’ who is Christ.” Jesus is the one in whom the promise is fulfilled; He is the offspring or seed of Abraham because He is everything that Israel was ever meant to be.
After Jesus’ resurrection, He met with His disciples and recommissioned them, breathing His Spirit into them. They became new men, new adams, remade in His image by the inbreathed Spirit of God. This, along with the greater outpouring at Pentecost some forty days later, was the beginning of a new humanity. He also restated the authority they had in His name, the wielding of the keys of heaven — loosing and binding, forgiving and withholding. The disciples were to go out, as Matthew records, to make disciples of all nations, carrying the name, authority and blessing to all so that in Christ and through His body, the church, all the families of the earth would be blessed.