“Behold, my covenant is with you, and you shall be the father of a multitude of nations.” – Genesis 17:4
This passage reiterates the covenant promise from the Lord when he called Abram out (Gen 12:2). God here affirms his plan to use Abram to launch a movement which will call God’s elect to himself.
Indeed, the passage is a reference to the biological and familial decedents of Abram, but a greater glory comes to the Lord in that he will call Abram’s spiritual offspring to himself. The promised offspring are those who are united with Abram in Christ. Both the Jews and the Gentiles who trust in Jesus are part of this promise.
In referencing this verse, Paul promises the Galatians that if they are disciples of Jesus, then they are part of the promises to Abram (Gal 3:29). Those who are in Christ are added to Abram’s family line and heirs of the promises God gave to Abram.
God declares Christians today were always considered part of his plan with Israel. The mouth of God, has then promised all Christians they will reach a multitude of nations. John Calvin said, “God declared that he was the speaker in order that absolute authority might appear in his words. Since our faith can rest on no other foundation than his eternal veracity, it becomes, above all things, necessary for us to be informed that what is proposed to us has proceeded from his sacred mouth.” The promise from God to us, his heirs, is that he will use us to reach the globe.
How glorious to know that we, as Christians, are not second class citizens or stepchildren to God. We are heirs of his promise to subdue the nations through us. This is no small matter. God desires to use his heirs to reach the nations with his perfect salvific gospel. The task to reach the nations is a great honor, but also, it is a great responsibility. It is a charge we must not disregard.
 John Calvin, Genesis, Crossway Classic Commentaries (Wheaton, IL: Crossway Books, 2001), 163.