The first reading at mass is hard for us to listen to. God seems to want human sacrifice. Even if in the end of the story the child is not sacrificed it still seems that the text presumes it would have been alright to go through with it.
The Old Testament relates a history of over two thousand years leading up to the birth of Jesus Christ. Over those two thousand years the understanding of the people of Israel both of God and how he wanted them to live developed slowly but dramatically.
There is evidence that at the beginning of that period it was indeed thought good to offer human sacrifice to God, of enemies captured in battle, and children. As Israel walked with God through the generations, they realised this was not what God wanted and the practice ended.
This story reflects that transition. Abraham starts out willing to sacrifice his child and by the close of the story he has learnt that God will provide all the sacrifice needed, he is the one who will pay the price of sin.
As well as echoing the transition away from human sacrifice in the history of the people of Israel, the story prefigures the ultimate sacrifice which brings about forgiveness and reconciliation: the sacrifice of Christ, which prepared for in Sunday's gospel reading when Jesus speaks to the Old Testament figures on top of the holy mountain about his “departure” his death.
Afterwards Jesus speaks to the disciples and talks also about his rising from the dead. They do not understand, but from that moment the journey to Jerusalem begins.
The account of the near sacrifice of Isaac is a reminder to us that sometimes we might find things scattered throughout the Old Testament which we think are not in keeping with what we know God wants. These are probably remnants of early beliefs which we should leave behind as the people of Israel learnt to do many centuries ago.