Abraham was the Father of faith. If we take the time to study the development of Faith, we will find that it is all about trust.
When Abraham and his family were in a time of famine he went to Egypt for food instead of turning to God. When he was in fear for his life he lied. His wife Sarah trusted him and yet she was given to Pharoah in exchange for Abraham;s life.. In fact Abraham lied twice about Sarah to save his own life. We can see that the level of trust in God, at this stage of his life was pretty weak. He then gave into Sarah's desire for a child through their maidservant, Hagar, even though they had been waiting years for their own child.
Abraham was not looking like a father of faith at this stage in his life.
So how did God get him from this place to a place of complete trust. A place in which he was prepared to take the life of his son, which he had waited all his life for.
This must show all of us that our faith in God is a journey and that no matter how we have failed, that with God in our lives there is always hope to change and develop into the person that God knows we can be.