Abraham's Ultimate Test

Now, I really do not get why God would ask Abraham to sacrifice the son that is suppose to build a great nation, but we cannot always tell why with Gods ways. I know the text says Gen 22:1-2 that God was testing him. But you know God already knew how Abraham felt. So could it be that maybe Abraham was getting too cocky about Isaac? Could it be that God wanted Abraham to prove to himself how much he really trusted God. I don't know.

My second thing I saw was the fact that God mentions Isaac in the text as Abraham's only son three times.
Gen 22:2 Take your son, your only son...
Gen 22:12 ...you have not withheld from me your son, your only son...
Gen 22:15 ...you have not withheld from me your son, your only son....
I really do not get this one. Remember Abraham has Ishmael too. What is the significance of God specifically mentioning your only son?

The one thing that strikes me on both these matters of why is the fact that God has a son that will be sacrificed hundreds of year later and God had only one son, Jesus Christ. So maybe this is not so much about why, but more about fulfilling the prophecy and for us to see how it all connects.