The verb for offering is used twice in this verse. The first time it is in the perfect tense as if Abraham indeed had sacrificed his son. But he did not do it, did he? No, but in his mind, he had already done it. By faith it was his intention to do what the Lord had commanded him. In Hebrews 11:17 it is written: “by faith Abraham offered up Isaac and” (then follows the second time this verb is used) he “was in the act of offering up his only son.” That means that he had already begun sacrificing.
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,