And they came to the place which God had told him of; and Abraham built an altar there, and laid the wood in order, and bound Isaac his son, and laid him on the altar upon the wood.
And Abraham stretched forth his hand, and took the knife to slay his son.
And the angel of the Lord called unto him out of heaven, and said, Abraham, Abraham: and he said, Here am I.
And he said, Lay not thine hand upon the lad, neither do thou any thing unto him: for now I know that thou fearest God, seeing thou hast not withheld thy son, thine only son from me.
And Abraham lifted up his eyes, and looked, and behold behind him a ram caught in a thicket by his horns: and Abraham went and took the ram, and offered him up for a burnt offering in the stead of his son.
~ Genesis 22:9-13

I am thankful I live in the post arrival of the the promised Messiah and not before. By Abraham's faith was he counted as righteous. He trusted God even before he had witnessed any benefits of doing so. Peter, witness to years of Jesus and his miracles, was told he would deny Jesus three times to which Peter vehemently said it couldn't happen. Fortunately, we have God's words, examples, and most importantly his Holy Spirit to guide us. As bad as these days are getting, I'm thankful for living now and all the help I can get.

I think why God commanded Abram to initially sacrifice his son was because, from the Chaldean culture (where he was raised) and the Canaanite culture (where he was going), child sacrifice was an acceptable practice to do in order to appease the gods and receive good fortune:
Psa 106:37-38 ESV - 37 They sacrificed their sons and their daughters to the demons; 38 they poured out innocent blood, the blood of their sons and daughters, whom they sacrificed to the idols of Canaan, and the land was polluted with blood.

Upon providing a replacement sacrificial ram, God then showed Abram that he was not like the gods of the surrounding nations, in order to bring Abram to a new religious understanding of what God required—obedience, not sacrifice:
Deu 12:31 ESV - 31 You shall not worship the LORD your God in that way, for every abominable thing that the LORD hates they have done for their gods, for they even burn their sons and their daughters in the fire to their gods.