Paul brings Scripture-proof for his position:
Related Commentaries of Galatians 3:6
Galatians 3:6-9
6 Even as Abraham believed God, and it was accounteda to him for righteousness.
7 Know ye therefore that they which are of faith, the same are the children of Abraham.
8 And the scripture, foreseeing that God would justify the heathen through faith, preached before the gospel unto Abraham, saying, In thee shall all nations be blessed.
9 So then they which be of faith are blessed with faithful Abraham.