The Joy of Sarah (Genesis 21:6)

"But God said to him, "Do not be distressed about the boy and your slave woman. Listen to whatever Sarah tells you, because it is through Isaac that your offspring will be reckoned." Genesis 21:12

King James Version reads in the part, "it is through  your Isaac that  your offspring will be reckoned", as follows:

"In Isaac shall thy seed be called." 

The word 'seed' (zera in Hebrew) refers to and is the same as the seed (i.e., the enmity between two seeds, that is, the seed of Satan and the seed of the woman, that is, Jesus, the Savior to come) found in Genesis 3:15. 

The word "called" in Hebrew (pronounced qara) is synonymous with 'call out, cry, or proclaim.'

Thus, the Lord's message to Abraham in effect is that "in Issac you can literally hear the cry of the baby to be born, that is, the Savior Jesus." Or "Literally Isaac (himself) proclaims the good news of the Savior to come." 

Isaac is the gospel carrier, the gospel of the good news of great joy for all mankind. 

The substance of the work of the gospel to be fulfilled through the Jesus Christ the Savior is reducible to one word: "One New Person out of the Two, that is Jews and Gentiles." The Lord revealed this vision to the Apostle Paul who said in effect, "The mystery of God is Jesus Christ, and the mystery of Christ is one new person out of  the two, that is, Jews and Gentiles." According to Paul all peoples on earth are categorized into three categories: Jews, Gentiles, and New Persons. 

Jews and Gentiles share one common characteristic, that is, unsaved. But for all who believe in the Lord, all Jews and Gentiles, everyone of them, are saved from their sins, and are born again into the family of God.

The name Isaac also incorporates the characteristics of the gospel, that is, total joy that comes from the fellowship with all that were once lost/estranged, but now found and restored back to the family of God, in perfect unity with God and with one another. When this happens the net effect would be, "Complete Joy" or "Full  Joy". 

Sarah expressed this hope and vision prophetically saying, "God has brought me laughter, and everyone who hears about this will laugh with me." Here 'this' refers to the birth of Isaac, the gospel carrier. The expression 'everyone' literally means 'everyone', the everyone who hears this good news and believes, has been saved into His Family, to the everlasting fellowship with the Father and the Son! 

I trust that Sarah's  testimony here in Genesis 21:6 came directly from God, foretelling the joy of people bursting into laughter,in total joy and peace, all in God's kingdom! 

It is in this hope of joy and vision that the Apostle Paul asked the members of the church in Ephesus to pray for him that words may be given him so that he will fearlessly make known the mystery of the gospel, namely one new person out of the two!