Gen 22:2
God said, "Take your dear son Isaac whom you love and go to the land of Moriah. Sacrifice him there as a burnt offering on one of the mountains that I'll point out to you."

God had promised Abraham this son and now told him to sacrifice the boy which shows you that what you can see and know is not everything! Just like with circumcision, you have to take what seems most valuable to you and give it to God and that is important to know.