I will pour my Spirit upon your offspring,
and my blessing on your descendants.
4 They shall spring up among the grass
like willows by flowing streams.
5 This one will say, 'I am the LORD’s,'
another will call on the name of Jacob,
and another will write on his hand, 'The LORD’s,'
and name himself by the name of Israel."
2 “I will make you into a great nation,
and I will bless you;
I will make your name great,
and you will be a blessing.
3 I will bless those who bless you,
and whoever curses you I will curse;
and all peoples on earth
will be blessed through you."
In Genesis 26, God confirmed it to Isaac thusly:
"...For to you and your descendants I will give all these lands and will confirm the oath I swore to your father Abraham. 4 I will make your descendants as numerous as the stars in the sky and will give them all these lands, and through your offspring all nations on earth will be blessed, 5 because Abraham obeyed me and did everything I required of him, keeping my commands, my decrees and my instructions.”
Then I looked ahead at the Jesus' Great Commission to His disciples in Matthew 28:
"...Go therefore and make disciples of all the nations, baptizing them in the name of the Father and the Son and the Holy Spirit, 20 teaching them to observe all that I commanded you; and lo, I am with you always, even to the end of the age.”
Next, consider what the Holy Spirit did to the disciples at Pentecost, in Acts 2:
When the day of Pentecost came, they were all together in one place. 2 Suddenly a sound like the blowing of a violent wind came from heaven and filled the whole house where they were sitting. 3 They saw what seemed to be tongues of fire that separated and came to rest on each of them. 4 All of them were filled with the Holy Spirit and began to speak in other tongues as the Spirit enabled them. 5 Now there were staying in Jerusalem God–fearing Jews from every nation under heaven. 6 When they heard this sound, a crowd came together in bewilderment, because each one heard them speaking in his own language. 7 Utterly amazed, they asked: "Are not all these men who are speaking Galileans? 8 Then how is it that each of us hears them in his own native language? 9 Parthians, Medes and Elamites; residents of Mesopotamia, Judea and Cappadocia, Pontus and Asia, 10 Phrygia and Pamphylia, Egypt and the parts of Libya near Cyrene; visitors from Rome 11 (both Jews and converts to Judaism); Cretans and Arabs–we hear them declaring the wonders of God in our own tongues!"
Consider that God gave the disciples this miraculous power so they could fulfill this central mission: to reach the ends of the earth. God poured out His spirit at Pentecost in a way that Isaiah prophesied: the disciples would take the Good News to lands which were dead to the knowledge of God. This is how God planned to fulfill His promise to Abraham and Isaac: through their descendents He would pour His spirit and bless the entire earth.
So on one hand the Great Commission is a command to get to work, but on the other hand it is a great comfort to know the reason for that is to share God's blessing which was originally given just to Abraham and the Jews, with the purpose to enrich the world. Comforted are we to be God's chosen people in adoption!
Postscript meditation, on Paul's letter to the Galatians, chapter 3:
26 So in Christ Jesus you are all children of God through faith, 27 for all of you who were baptized into Christ have clothed yourselves with Christ. 28 There is neither Jew nor Gentile, neither slave nor free, nor is there male and female, for you are all one in Christ Jesus. 29 If you belong to Christ, then you are Abraham’s seed, and heirs according to the promise.