Galatians 4:28 identifies believers as children of promise like Isaac, directly contrasting the flesh-born son of Hagar here in Paul's allegory.

In Genesis 17:15-19, God promises that Sarah will bear Isaac, the son of the promise — this is the specific promise Paul contrasts with Ishmael's birth according to the flesh.

In Genesis 18:10-14, the Lord reiterates the promise of Isaac to Sarah, who laughs — this reinforces the miraculous, promise-based nature of Isaac's birth.

Genesis 21:1 records God fulfilling His promise by visiting Sarah — this is the actual event that Paul refers to as Isaac being born through promise.

Genesis 21:2 gives the specific birth of Isaac by Sarah in her old age — this is the concrete fulfillment of the promise Paul highlights.

In Romans 4:18-21, Paul emphasizes Abraham's faith in the promise against all odds — this faith underlies the 'son of the free woman' born through promise.

Romans 9:7 quotes 'Through Isaac shall your offspring be named,' directly affirming that the promise runs through Isaac, not Ishmael.

Romans 9:8 explicitly states that children of the promise, not of the flesh, are counted as offspring, matching the contrast in Galatians 4:23.

Hebrews 11:11 highlights Sarah's faith in God's promise to conceive — this directly explains the faith behind Isaac's birth 'through promise'.

Genesis 16:15 records the birth of Ishmael by Hagar — this is the 'son according to the flesh' that Paul contrasts with Isaac.