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 By faith Abraham, when he was tested, offered up Isaac, and he who had received the promises was in the act of offering up his only son,