Many times we see a dual fulfillment of prophetic texts. For example, regarding the promise to Abraham (Abram) that God would make him a great nation.
Keith Mathison writes (https://www.ligonier.org/blog/promise-abraham-unfolding-biblical-eschatology):
“In Genesis 12:2, God promises that he will make of Abram “a great nation.” This promise will be fulfilled initially in the birth of the nation of Israel. This promise necessarily implies that Abram will have offspring, but like the promise of land, the promise of offspring is only made explicit when Abram reaches Canaan (cf. 12:7). The promise of offspring is also related to God’s ultimate kingdom purposes. Just as the land promise provides a realm for God’s kingdom in the midst of his creation, the promise of offspring anticipates a people for his kingdom.”
1. The initial fulfillment regarding the sons of Abraham is the physical descendants of Abraham, Isaac, and Jacob, the ethnic nation of Israel. (Note: Sojourners and proselytes were also included in the nation of Israel)
In the new testament, Paul reveals a stunning truth: The phrase, “sons of Abraham,” does not allude only to the physical descendants, but also to a group of spiritual descendants, counted righteous by faith (Galatians 3:28-29; Romans 4:11), like the man of faith, Abraham.
2. The second, or, ultimate fulfillment of the promise, refers to all the spiritual sons of Abraham: Jew and gentile; all who are counted righteous by faith.
If there are natural sons of Abraham and spiritual sons of Abraham, does not Scripture also bear out the mystery, that there is not only a natural Israel (the twelve tribes of Israel) but also a spiritual Israel (made up of Jew and Gentile), who are longing for the city to come (Hebrews 11:10;13-16; 13:14), who comprise the bride of Christ (Revelation 19:7-8), who will serve as a royal priesthood in the coming kingdom of God (Revelation 1:5-6; 5:9-10), and receive and possess the kingdom for ever and ever (Daniel 7:27)?
Could this spiritual Israel, the ultimate fulfillment of God’s promise to Abraham, be described as the “Israel of God” (Galatians 6:16)?