Now the promises were spoken to Abraham and to his seed. He does not say "and to seeds," as though referring to many, but and to your seed, referring to one, who is Christ.Galatians 3:16(HCSB)
The unconditional covenant promise that was made to Abraham was the same promise that was made to Christ. Jesus Christ was the Seed to Whom this Abrahamic promise was made: "Now to Abraham and his Seed (the Lord Jesus Christ) were the promises made. He saith not, and to seeds, as of many; but as of one, and to thy Seed, which is Christ."
Abraham was saved by grace through faith, and God credited it to him as righteousness. The promise to Abraham, that all the families of the earth would be blessed through him, was to come about through his unique Seed, Christ Jesus our Lord, for it is through His sacrificial death and glorious Resurrection that all the peoples on earth would be blessed, both Jew and Gentile.
Many centuries would have to pass before that promised Seed would be born as a baby in Bethlehem. Much time would have to elapse before the promised Blessing which God had pledged to this father of faith, would came to tabernacle among men and give His life as a ransom for many so that through Him, all the families of the earth would be blessed and many would be made righteous, by faith.
It is only by means of the death, burial, and Resurrection of Christ that this very specific, Abrahamic promise (that all families of the earth will be blessed) could come to pass. For God loved the world so much that He gave His only begotten Son, the promised Seed of Abraham, to die for the sin of the whole world so that whosoever believes in Him would not perish but be blessed with the forgiveness of sins and life everlasting.
Other glorious, unconditional promises were also made to this great patriarch of Israel, for God also promised to bless His people Israel: "In multiplying I will multiply thy seed as the stars of the heaven, and as the sand which is upon the sea shore; and thy seed shall possess the gate of his enemies."
Jesus Christ is the Seed to Whom this particular, staggering Abrahamic promise of world-wide blessing was made, for only the eternal Son of God and perfect Son of Man (Who is the single Seed of Abraham) is able to save His people Israel from their sins, to die for the sin of the whole Gentile world, and to unlock that precious promise that God made to His forefather: "That all the families of the earth will be blessed."
It was to Abraham that this staggering promise was made. It was to Abraham and his Seed, Christ Jesus our Lord: "For God did not say, 'and to seeds, as of many - but as of One - and to thy SEED, which is Christ.'"
Heavenly Father, thank You that in Christ, the promise given to Abraham has been realised, and that in Christ, both Jew and Gentile, young and old, male and female, bond and free, are blessed through Him, for He took upon Himself the sins of the whole world so that whosoever believes in Him should not perish but have everlasting life. To Him be all praise and glory, forever and ever, AMEN.