Now to Abraham and his seed were spoken the promises. He does not say, “And to seeds,” as upon the basis of many, but as upon one. “And to your seed,” who is Christ, Galatians 3:16.
Paul is referring to the covenant God made with Abraham in Genesis 22:16–18. Abraham did not withhold his son from God when God asked him to sacrifice him. This request had nothing to do with actually sacrificing his son; it was to display Abraham’s faith. Abraham, when he was tested, offered up Isaac, the son of promise, concluding that God would raise him up from the ashes (Hebrews 11:17–19). Abraham’s actions displayed his faith in God’s promise.
Because Abraham did not withhold his son, God gave him a covenant that does not relate to the two covenants previously given—the first being land upon this earth (Genesis 15), and the second, land in eternity (Genesis 17). This covenant concerned a single descendant who would be plural in effect.
“Blessing I will bless you, and multiplying I will multiply your descendant as the stars of the heaven and as the sand which is on the seashore; and your descendant shall possess the gate of his enemies” (Genesis 22:17).
Seed is singular in the original, not plural as it is translated in many English versions, due to the confusion of translators concerning the use of a singular seed that would be as the stars of the heavens.
When God raised Christ from the dead, He created a new man (2 Corinthians 5:17). Into this new creation all grace believers are immersed, making the singular descendant as the stars of the heavens and the sand of the seashore.