His son as an offering was considered such a great act. If Hashem would command any person to do such a thing, who would ever refuse? We know that Hashem could take any life He wants at any time, so what would be the point of saying no?
Sefer Kometz Haminchah answers that Hashem told Avrohom to “please” (“nah”) take his son, and Chazal say that the word “nah” always indicates a “bakasha” (request). This means that Hashem did not openly command Avrohom to sacrifice his son. Rather, he merely requested him to do it. He told him that this is what He wanted him to do, but He would not punish him if he didn’t do it.
This means that Avrohom had the free will to decide if he wanted to fulfill Hashem’s will or not. He also could have argued with Him or davened to save his son. But Avrohom didn’t do any of those things. He got up early in the morning and hurried to do fulfill Hashem’s will.
This was, in fact, a very difficult test. Avrohom had free will and it was fully up to him, yet he enthusiastically chose to fulfill Hashem’s request.
