Isaac: A Funny Foundation (Genesis 21:1-21)

Reformed & Expository Preaching

Genesis 21 recounts the fulfillment of God’s long-awaited promise to Abraham and Sarah—the birth of Isaac. This chapter centers on God’s faithfulness, the nature of grace, and the surprising ways God accomplishes His purposes.

The chapter opens with the Lord visiting Sarah, just as He had promised (v. 1). At the age of 90, Sarah gives birth to Isaac—a name meaning “laughter”—as a testament to the joy that comes when God’s promises are fulfilled, even in the face of human cynicism.

Isaac shows the Lord’s redemption: God moves his people from cynical laughter to joyful celebratory laughter. God’s promises don’t operate on our timeline; they operate on His. Isaac’s arrival reminds us that waiting on God is not wasted time but formative time—teaching trust in His sovereignty.

Isaac’s arrival contrasts sharply with Ishmael’s, Abraham’s son through Hagar. Ishmael represents human effort—Abraham and Sarah’s attempt to secure God’s promise through their plans. Isaac, however, is entirely the result of divine grace, showing that God’s blessings come as gifts, not as something we can earn or achieve.

Isaac shows the impossibility of the Christmas promise. The thought of the Lord establishing his promise through death seems absurd. However, Abraham and Sarah testify that the Lord establishes life through death because the Lord knows the depth of sin, and what it will take to overcome it. Isaac sets the joyful foundation, but Christ is the one who definitively establishes the foundation by fulfilling God’s promise.