Did God tell Abraham it was a test? Or did God let Abraham think God really intended him to kill his son?
How much of a test would it have been had not God let Abraham falsely believe he must sacrifice his son?
Of course He didn't. It wouldn't be much of a test if Abraham had it all laid out for him. As far as he was concerned, Abraham was just obeying something that God commanded him to do.
Or did God let Abraham think God really intended him to kill his son?
It certainly is a valid question and one for which one can only speculate as to what was going through Abraham's mind. No doubt God's request threw Abraham for a loop because so many ends of the questions running in his mind must have seemed that they just wouldn't come together and make sense. For example, Abraham had repeatedly received a promise directly from God concerning his descendents and because he and Sarah were quite old by the time Isaac was born, it seemed to them to be physically unlikely they would have any more children i.e. He was getting on to 75 when the promise was first made and 100 when Isaac was finally born.
Genesis 12:2 'And I will make of thee a great nation, and I will bless thee, and make thy name great; and thou shalt be a blessing:'
Genesis 17: 4 'As for me, behold, my covenant is with thee, and thou shalt be a father of many nations. 5 Neither shall thy name any more be called Abram, but thy name shall be Abraham; for a father of many nations have I made thee.'
Genesis 18:10 'And he said, I will certainly return unto thee according to the time of life; and, lo, Sarah thy wife shall have a son....... 17 And the LORD said, Shall I hide from Abraham that thing which I do; 18 Seeing that Abraham shall surely become a great and mighty nation, and all the nations of the earth shall be blessed in him?'
Throughout this entire period, Abraham may have been impatient but it does not appear that he ever lost his faith that God would do what he was going to do - and that includes what went on after the request was made by God to sacrifice his son. This is borne out in the account where Abraham finally was heading off to the mountain to sacrifice Isaac.
Genesis 22: 5 'And Abraham said unto his young men, Abide ye here with the ass; and I and the lad will go yonder and worship, and come again to you.'
Why would Abraham say this to his servants if he thought that Isaac would not be returning with him from the mountain? Even at this point in the narrative, it appears that Abraham was showing that he had total faith in God and whatever the plan was that God was about to unfold. My guess is that Abraham was thinking that he would sacrifice Isaac but God would miraculously raise his son up again.... but since we don't know anything beyond what is written, it is just speculation. One thing that is apparent is that Abraham passed the test.