God promised Abraham that through him all people groups of the world would be blessed. This blessing was the coming Messiah who would one day provide the way of salvation for all people groups.
God’s covenant with Abraham, “I will make of thee a great nation,” was fulfilled in three ways.
(a) In a natural posterity – “as the dust of the earth,” Gen. 13:16; John 8:37 namely the Hebrew people.
(b) In a spiritual posterity – “look now toward heaven… so shall the seed be” (John 8:39); (Rom 4:16-17); (Rom 9:7-8); (Gal 3:6); (Gal 3:7); (Gal 3:29), namely all men of faith, whether jew or gentile.
(c) fulfilled also through Ishmael (Gen 17:18-20).