Why does the Messiah have to be God?

Mostly I'd ditto Eric's answer.

Let me add: COULD God have carried out his plan with a human being whom God specially created or ordained to be sinless, or special in whatever ways required? I don't know. You could ask all sorts of hypotheticals like that. Could God have saved humanity without sending Jesus at all? Could he have created a universe in which murder was not a sin? How about one where there was no such thing as electricity, or the color blue? Some such hypotheticals are easy to imagine, others not so much.

The Bible says that Jesus was God. Whether he "had to be" for salvation to be possible ... Could God have come up with some other plan? I don't know.

According to Isaiah, the Messiah was a man- namely Cyrus. (Isaiah 45:1) Thus, if we are to read the text literally, the Messiah was in fact man.

The salvific purpose of the Messiah was also temporal. As the Jews understood it, he would bring freedom to the captives, raise the poor from their situation, and generally turn the order upside down.

What only God could do was redeem people from the pit and the grave.

While I'm not sure that the Messiah had to be God, the fact that Jesus was the son of God is attested to in the Scriptures. The nature of Jesus, of course, is that he is fully God and fully man.

As Isaiah 49 says, it is "too little a thing that my son should merely save Israel."

That move - from Messiah to the eternal plan effected in His Sonship is where I believe the necessity of the Divine comes into play.

The premise of the question is incorrect. The Messiah does not have to be God. Have a look at this Wikipedia article for a long list of messianic claimants:

Jewish Messiah claimants

All of these figures were thought by some to be the messiah at one time. But none of them were spoken of in the kinds of terms used about Jesus of Nazareth. That doesn't in itself "prove" something about Jesus. It does show that a claim to be Messiah is not equivalent to a claim to be any sort of deity.

So the real question isn't why the messiah had to be God (as he didn't), but why Jesus was thought of as the Son of God and not simply as the Messiah.

Along with that, note that most of the messianic movements in the first century dispersed when their leader was killed. The same happened to the Jesus movement, but that movement obviously did not disperse when its leader was killed. (Why? Well, as a Christian, it should be obvious what my theory is.)

I was planning on an answer almost as Eric's which will now carry no weight so I'd focus on possible consequences if man had played Jesus' role.

First of all I do not think we could compare man's 'perfection' (Mary or anybody else) to that of God.

"There was a man in the land of Uz, whose name was Job; and that man was perfect and upright, and one that feared God, and eschewed evil." (Job 1:1).

Does this mean Job was sinless throughout his life? This is what our bible says about him, how much more a doctrine created by a 'brand' of christianity, a belief that man thinks reflects what is in the bible.

As righteous as man could be, he could not understand FULLY the plans of God. We know what God's main purpose is but we do not understand all his ways, methods and even time of accomplishments. Let's get back to Job for instance. He faced harsh times but...

"In all this Job sinned not, nor charged God foolishly." (Job 1:22).

No doubt about his faithfulness to God but he began to degrade his significance under the eyes of God. Though he did not charge God foolishly, there were wild and deeper complaints that pushed God to remind him of who he (God) is.

"Then the LORD answered Job out of the whirlwind, and said, Who is this that darkeneth counsel by words without knowledge? Gird up now thy loins like a man; for I will demand of thee, and answer thou me." (Job 38:1-3).

After a long, strong reply from God, Job confesses:

"Who is he that hideth counsel without knowledge? therefore have I uttered that I understood not; things too wonderful for me, which I knew not." (Job 42:3).

When we know that His ways are not our ways, it tells us that our worldly desires (by default) cannot allow God's will to totally be done on earth, even when it is our prayer. Even Moses, whom I consider as one of the most active and closest prophets of God, sometimes lacked boldness, and failed to carry out 'simple orders', not to mention Jonah.

Can we compare these to Christ (God)? When the time for all the physical torture to begin, Jesus felt the heaviness,

"And being in an agony he prayed more earnestly: and his sweat was as it were great drops of blood falling down to the ground." (Luke 22:44).

yet He asked His will to be done.

"He went away again the second time, and prayed, saying, O my Father, if this cup may not pass away from me, except I drink it, thy will be done. " (Matthew 26:42).

His death was not a one time killing by bomb blast which others (martyrs) will claim to die in for their course. It isn't a gunshot that will limit the period of pain before death. This One is spat upon, whipped, crowned with thorns, among others, finally hanged by nails on a cross and left to die.

For humans, the thought and expectations alone, knowing that all these will happen are torture enough to make us NEED an escape from such fate. We may be tempted to argue with God: Why at all must I suffer all these before I die, why not death by the sword in the garden after Peter reacts? And a whole lot, simply because even when we believe in Him, we cannot comprehend fully and allow God's will to be done without questions. Only He could.

An interesting opinion I heard on this (and incidentally the reason I asked the question, though I hoped for and got better answers) had to do with the infinitude of God.

Since God is infinite, sinning against him is an "infinite" sin (for those who believe in "trivial" sins, assume i'm talking about a lifetime of sin from someone). For God to be just, infinite sin requires infinite payment. That's why we say it's fair for God to punish people infinitely in Hell for only a finite amount of sin.

Now, the sacrifice would be taking the place of, not one, but millions (presumably, who knows how many will be saved) of people, all who have lived their entire lives sinning against an infinite God.

So not only did Christ need to be sinless to be a representative, he needed to be a viable candidate. A human representative would have to actually go through Hell for infinity, and then do it again, and again - suffering hell for infinity for each and every sinner. Since this is impossible, we would need one representative for each of us - and each representative would have to be perfect, and would suffer in Hell forever.

Instead, God sent his Son, incarnated him as man. The Godman, as Eric said, could both represent man, and was both sinless and infinite. As a result, he could suffer the infinitude of Hell for each one of us. The punishment was no less terrible! If anything, it was worse! But for the so many hours he was on the cross (or the three days he was in the tomb; depending on how you see it) the eternal son of God become man suffered infinite punishment for sin against himself.

And that, I think, is why the Christ had to be God.

I think that there are two main reasons why the Messiah had to be God.

One, which you alluded to in your question, is that he had to be sinless in order to be the perfect sacrifice for sins. I don't hold that Mary was sinless (Romans 3:10), and I believe that only God could have been sinless. So in my opinion, that is a valid line of reasoning for the necessity of the Messiah to be God.

The second reason, is that the Messiah needed to be God for the same reason that he needed to be man. Hebrews tells us why Jesus had to become man:

Hebrews 2:17-18 (NIV)
¹⁷  For this reason he had to be made like them, fully human in every way, in order that he might become a merciful and faithful high priest in service to God, and that he might make atonement for the sins of the people. ¹⁸  Because he himself suffered when he was tempted, he is able to help those who are being tempted.

Hebrews 4:14-15 (NIV)
¹⁴  Therefore, since we have a great high priest who has ascended into heaven, Jesus the Son of God, let us hold firmly to the faith we profess. ¹⁵  For we do not have a high priest who is unable to empathize with our weaknesses, but we have one who has been tempted in every way, just as we are —yet he did not sin.

The Messiah needed to save mankind, and in order to do so, he needs to act as a high priest, an intermediary between God and man. These verses show us that Jesus needed to become man in order to empathetically mediate for man. I suggest that he also needed to be God, in order to mediate to God. Jesus acts as a bridge between God and man, by having both natures.

Because Jesus is in nature God, we have a perfect high priest in the heavenly tabernacle. This provides for us an anchor behind the veil (Hebrews 6:19), which is far superior to any merely human priest. Hebrews 7 explicitly states the flaws in having human priests, and why a priest with the eternal nature of God is far superior:

Hebrews 7:23-25 (NIV)
²³  Now there have been many of those priests, since death prevented them from continuing in office; ²⁴  but because Jesus lives forever, he has a permanent priesthood. ²⁵  Therefore he is able to save completely those who come to God through him, because he always lives to intercede for them.

This is contrasting the disadvantages of human priests with the advantages of having God as our high priest. The writer of Hebrews then goes on to list some characteristics of such a high priest that meets our needs:

Hebrews 7:26 (NIV)
²⁶  Such a high priest truly meets our need—one who is holy, blameless, pure, set apart from sinners, exalted above the heavens.

In order to intercede for us to God, the writer of Hebrews tells us that this high priest needed to have many characteristics that we would only associate with God: holy, blameless, pure, set apart from sinners, exalted above the heavens.

Even if we accepted the sinlessness of Mary, I don't think anyone would attribute to her these necessary characteristics of an effective heavenly high priest (except perhaps pure and blameless).