Know then that it is those of faith who are the sons of Abraham.  And the Scripture, foreseeing that God would justify the Gentiles by faith, preached the gospel beforehand to Abraham, saying, “In you shall all the nations be blessed.”  So then, those who are of faith are blessed along with Abraham, the man of faith.
When God said that through Abraham all the nations of the earth would be blessed, He meant that people from all the world would be people of faith in Jesus Christ and therefore be children of Abraham. The promise to Abraham that he would the Father of a great nation was never intended to stop with Israel. God meant that this great nation that Abraham would father, would be a people from all the nations and the citizenship would be based on faith, not on geography or place of birth.
For the Galatian believers to hold on to the Law, is for them to go back to that which was a shadow of better things to come (Hebrews 10:1). The political entity known as Israel is not the chosen nation of God in the sense that God only has plans to save Jews.
The fulfilment of the promises to Abraham and Israel are fulfilled by Christ through the church and that was always their intention. Every time a person comes to faith in Christ the promise God made to Abraham is being fulfilled. Christians can read their Old Testaments without having to believe that Israel as a political entity is still awaiting the fulfilment of the promises made to the patriarchs.
Christians, both Jew and Gentile, should rejoice in the fact that they are the product of a plan forged before the foundations of the world and is now being realized. That plan came into being through Israel for the blessing of all the nations. God still has a special place in His plan of salvation for Jewish people (Romans 11:1-4). That special place is based on the grace of God through faith in Jesus Christ (Romans 11:5, 25-26).
Its hope is far greater than a piece of geography. We are all Israel who are of faith in Jesus Christ and that is cause for great rejoicing.