Messiah has to be God
Atonement was necessary to reconcile sinners to God. God is holy and required a perfect sacrifice to effect atonement. At first the people sacrificed animals, but the blood of bulls and goats could not cleanse, it could only cover sin. Thus God promised that He would provide an acceptable atonement.
Genesis 22:6-8 (also Deuteronomy 32:43; Ezekiel 16:63)
Abraham took the wood of the burnt offering and laid it on Isaac his son; and he took the fire in his hand, and a knife, and the two of them went together. But Isaac spoke to Abraham his father and said, “My father!” And he said, “Here I am, my son.” Then he said, “Look, the fire and the wood, but where is the lamb for a burnt offering?” And Abraham said, “My son, God will provide for Himself the lamb for a burnt offering.” So the two of them went together.
In fact He said that HE would be the atonement, for no mere mortal is without sin.
Isaiah 63:7-9 (Also Isaiah 44:6; 63:8-9 and 1 Peter 1:18-19)
I will mention the loving kindnesses of the Lord and the praises of the Lord, according to all that the Lord has bestowed on us, and the great goodness toward the house of Israel, which He has bestowed on them according to His mercies, according to the multitude of His loving-kindnesses.
For He said, “Surely they are My people, children who will not lie.” So He became their Savior.
In all their affliction He was afflicted, and the Angel of His Presence saved them; in His love and in His pity He redeemed them; and He bore them and carried them all the days of old.