Throughout Genesis 12-17, God made many promises to Abraham. These promises were not just to Abraham, but they were also to his descendents. For instance, in Genesis 12:7, God promised, “To your descendants I will give this land.”
The Hebrew word for “descendants” is zerah. The literal translation of zerah is “seed” or “offspring” or “descendant.” This is a collective noun, which means that even though it is singular, it could be referring to a group.
From Genesis 12-17, it is clear that zerah should be understood collectively. For instance, in Genesis 15:5, God says, “Look now toward heaven, and count the stars if you are able to number them. So shall your descendants be.” Just as there are a plurality of stars, so Abraham will have a plurality of descendents.
Paul picks up this concept in Galatians 3 in his discussion of the “seed” of Abraham. Paul makes this startling statement in Galatians 3:16,
“Now to Abraham and his Seed were the promises made. He does not say, ‘And to seeds,’ as of many, but as of one, ‘And to your Seed,’ who is Christ.”
Paul argues that because zerah is singular, it should be interpreted as such and not collectively. Jesus Christ is the true zerah of Abraham.
Thus, what is important is not physical descent from Abraham. What is important is spiritual descent from Abraham and ultimately, Abraham’s seed, Jesus Christ.
This is Paul’s point in Galatians 3:29, “And if you are Christ’s, then you are Abraham’s seed, and heirs according to the promise.”
Gentiles who have no physical relationship with Abraham can become a descendent of Abraham if they are spiritually related to Jesus Christ.