His name has an article: he is the (well‑known) Isaac. He is the only son. Abraham had another son (Ishmael) but Isaac was chosen by God. Isaac is the only son (Hebrews 11:18) of whom was said, “Through Isaac shall your offspring be named” (see Genesis 21:12; Romans 9:7). That makes the test of Abraham’s faith so difficult, almost unimaginable: Isaac was his only son; through Isaac, God would fulfill his promises—and then kill him?! Read Genesis 22:1–24.
17 C'est par la foi qu'Abraham offrit Isaac, lorsqu'il fut mis à l'épreuve, et qu'il offrit son fils unique, lui qui avait reçu les promesses,