Adam Clarke's Commentary on the Bible

Verse 17 (Hebrews 11:17)

Abraham, when he was tried - See the history of this whole transaction explained at large in the notes on Genesis 22:1-9 .

Offered up his only-begotten - Abraham did, in effect, offer up Isaac; he built an altar, bound his son, laid him upon the altar, had ready the incense, took the knife, and would immediately have slain him had he not been prevented by the same authority by which the sacrifice was enjoined. Isaac is here called his only-begotten, as be was the only son he had by his legitimate wife, who was heir to his property, and heir of the promises of God. The man who proved faithful in such a trial, deserved to have his faith and obedience recorded throughout the world.

- Adam Clarke's Commentary on the Bible