Why did God test Abraham and ask him to sacrifice his son?
In asking Abraham to sacrifice his son, God was not testing him for his own information but for that of Abraham. Up until this point Abraham did not know that he had such faith within himself. In making such a difficult request, God actually drew the best out of Abraham. This is why he gives each of us the crosses we have.
God had big plans for Abraham, who was to become the father of many nations and most of all our "father in faith" as our liturgy proclaims: “Look with favor on these offerings and accept them as once you accepted the gifts of your servant Abel, the sacrifice of Abraham our father in faith, and wine offered by your priest Melchisedech.”