Genesis 12:3 in the Light of Galatians 3:7, 14

Galatians 3:14 says, "This was so that the blessing of Abraham would come to the nations by means of Christ Jesus, so that we might receive the promised spirit through faith [in Christ Jesus]."

Genesis 12:3 applies as a promise of blessing for those who are the real sons of Abraham (see Galatians 3:7). Those of that description, and who were living before God's judgment was executed against the Jewish nation in 70 C.E. by the Roman armies, were not restricted to only the ones who had been circumcised out of obedience to the Law of Moses. No, but included among them were uncircumcised (Gentile) believers from among the nations, because for those in the new covenant, which charters the real Israel of God, neither is circumcision of any moment with God, nor is uncircumcision of any moment with God (see Galatians 5:6; 6:15, 16). Those, however, who are in the new covenant, and who own membership in the Abrahamic covenant, cannot have received the promised anointing of holy spirit apart from faith in Christ Jesus, for faith in Christ Jesus is necessary for anyone -- Jew or Gentile -- to own fine relationship with the God of Abraham. 

Genesis 12:3 will be read by true Christians in the light of the teachings of Jesus and his apostles as recorded in the Bible. So, we see in Galatians 3:14 that the blessing of Abraham (Genesis 12:3) does not come to those who believe they must bless (work for, support) the Jewish polity in order to receive the blessing of Abraham. No, but for those who receive the blessing of Abraham, they receive it only by means of Christ Jesus.

There is no antisemitism in the Christian, Biblical doctrine of supersessionism rightly understood. True Christians out of love for Jewish people preach among and seek to teach Jewish people the Gospel (the Good News of the Kingdom of God) wherever Jews are found. Nobody alive today -- whether Jew or Gentile -- can be a friend of God like Abraham was unless he has a living faith like Abraham's, which since Jesus' earthly ministry must include Christian discipleship.