Sarah conceived, and bore Abraham a son in his old age, at the set time of which God had spoken to him.
⛓ Read Genesis 21 forward — the whole chapter and its afterlife →Where this passage surfaces again, sorted by the strength of the link. Each kind of reuse — formal quotation, deliberate allusion, faint echo, figural pattern — carries its own mark. Follow any reference to read it in place.
Allusion1
A deliberate verbal borrowing without a citation formula.
The fulfillment paired with the promise of 17:19: Sarah bore Isaac ‘when she was past age.’ Low verbal volume, but high coherence — the late-life birth is the very ground of the ‘as good as dead… stars and sand’ paradox in v. 12.
Medium confidence◇ Figural2
A typological pattern re-read in a later text — no verbal quotation, high figural coherence.
'One by the free woman' born 'through promise' (vv.22-23) recounts Sarah bearing Isaac at the appointed time God had promised. Volume and coherence are maximal; Paul builds the free-woman/promise half of the allegory directly on this Genesis event.
High confidence'We, brothers, as Isaac was, are children of promise' (v.28) identifies believers with Isaac, the son born by promise (Gen 21:1-2). Coherence with the promise/flesh allegory is exact; Isaac is the explicit figural type.
High confidence