But he did receive the sign of circumcision. So did every one of his male decendants. Also, take a look at what he was doing when he was “saved by faith.”
Was he thinking, “Hey, wouldn’t it be nice to believe that there is One True God who is our Creator? I think that from now on, I will believe that this is so.” No - he was making a blood sacrifice to the One True God.
He figured out (by God’s grace) that God existed, but that wasn’t the end of it for him. Once he figured out that God existed, he responded in faith by doing an action - the sort of an action that one does in response to a God - his “faith alone” was to make a ritual sacrifice to God, to see what transpired. God indeed heard him, and gave him the covenant of circumcision.
Do you think that if Abraham had just sat there on his bed thinking “Yes, there is a God” and then just rolled over and gone back to sleep, without making any kind of response to this information other than “just believe it”, that he would have received the covenant of circumcision?
Abraham also shows us that we need to respond to what we know by faith - that we need to act on it, and take it to the next level.