"Therefore sprang there even of one, and him as good as dead, so many as the stars of the sky in multitude, and as the sand which is by the sea shore innumerable." (Hebrews 11: 12)
This verse speaks of you and me, Beloved. We are all sons of Abraham by faith in Christ Jesus.
For a little background, consider once again the promises of a Deliverer for us:
"And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel." (Genesis 3:1 5)
"Therefore the Lord himself shall give you a sign; Behold, a virgin shall conceive, and bear a son, and shall call his name Immanuel.: (Isaiah 7: 14)
Notice that God prophesied about "her seed" and a virgin giving birth to a child, all of which speaks of Jesus:
"Then said Mary unto the angel, How shall this be, seeing I know not a man? 35And the angel answered and said unto her, The Holy Ghost shall come upon thee, and the power of the Highest shall overshadow thee: therefore also that holy thing which shall be born of thee shall be called the Son of God." (Luke 1: 34-35)
Jesus was born king of the Jews, and yet He became sin to make us all the righteousness of God in Himself (2 Corinthians 5: 21).
Through His death and resurrection, we receive life and the Spirit of Adoption, sons of the Most High God:
"4But God, who is rich in mercy, for his great love wherewith he loved us, 5Even when we were dead in sins, hath quickened us together with Christ, (by grace ye are saved;) 6And hath raised us up together, and made us sit together in heavenly places in Christ Jesus:" (Ephesians 2: 4-6)
"For ye have not received the spirit of bondage again to fear; but ye have received the Spirit of adoption, whereby we cry, Abba, Father. " (Romans 8: 15)
"And because ye are sons, God hath sent forth the Spirit of his Son into your hearts, crying, Abba, Father. 7Wherefore thou art no more a servant, but a son; and if a son, then an heir of God through Christ." (Galatians 4: 6-7)
Paul summarizes why the Barren Woman in Isaiah 54 can rejoice:
"7For it is written, Rejoice, thou barren that bearest not; break forth and cry, thou that travailest not: for the desolate hath many more children than she which hath an husband.