That Christ died for our sins is beyond question. (1 Cor. 15:3; Rom. 5:8-11; 2 Cor. 5:21) In what sense was Jesus’s death for sin necessary, though? The issue of the necessity of atonement is a complex one involving many layers from various angles. John Murray sheds some light on at least one area by making a very helpful distinction in his work Redemption: Accomplish and Applied between two views of the necessity of Christ’s work on the Cross that have been held throughout the history of theology.
Hypothetical Necessity – First is the view that he terms “hypothetical necessity” (pg. 11), in which Jesus’ death is held to be not strictly necessary. Theoretically it is possible that God could have saved his church through some means other that Christ’s sin-bearing death, and victorious resurrection if he so chose. He is God for whom all things are possible. Apparently in his wisdom though, he found that this was the most fitting means in that it combines the greatest amount of blessings, virtues, and so forth. Murray cites Augustine and Aquinas as historical representatives of this view. Knowing what I know of them, it sounds like the sort of conclusion they might come to.
Consequent Absolute Necessity – The second view Murray calls “consequent absolute necessity” and it is apparently the historical Protestant position (Calvin, Institutes, II.16.5). It holds that, given God’s decision to save, Christ’s death for sin was absolutely necessary. So, to be clear, first, it affirms that strictly speaking, Christ’s death is unnecessary. God did not need Christ to die. He is perfect and complete in his own life before the creation or the redemption of the world without it. And yet, consequent, or logically-following his decision to save, it is absolutely necessary given the nature of sin and the nature of God that it should happen through Christ’s vicarious sacrifice. (pg. 12) So, if moved by love, God is going to save people in a way consistent with his own holy, just, and merciful character, the atonement is necessary. It is this view that Murray judges to be the correct one and to which I myself subscribe.
Support Some might find this to be ‘vain speculation’, but Murray points various texts to the effect that such a conclusion is warranted. (See pp. 13-18) One particular argument he forwards is connected to the fact that the Scriptures point to Christ’s death as greatest proof we have of the love of God (Rom. 5:8; 1 John 4:10) That God was willing to spare his own Son testifies to the costliness of his love. (Rom 8:32) Murray asks, “would the Cross be an extreme exhibition of love if there were no necessity for such costliness?” (pg. 17) Can’t we only draw that conclusion if there were no other options? If there wasn’t some extreme sense of necessity involved? I find that conclusion hard to escape. As another has pointed out, it makes no sense for your friend to tell you, “Look how much I love you!” and then jump into a lake and drown. If he jumps in front of a bus to push you out of the way, that’s another story.
Another theological objection is that this seems to put limits on God. Isn’t it arrogant to say what God can and can’t do? At one level yes, we ought to be careful about being too eager to say what God can’t or cannot do. That’s why Murray spends several pages showing that this is a judgment made on scriptural grounds; its safest to make judgments on what God has told us about himself. It must also be pointed out that there are several things it is perfectly fine to say God’s ‘cannot’ do which do not rob him of glory. God’s inability to lie, be wicked, or deny himself is no affront to his majesty. In fact, “such ‘cannots’ are his glory and for us to refrain from reckoning with such ‘impossibles’ would be to deny his glory and perfection.” (pg. 13)
So was Jesus’ death necessary? In one sense, no. Jesus didn’t need to die–he could have left us to our peril and woe. In another sense, once God decided (in some pre-temporal sense) to save us by grace, it could come in no other way than through the death of His Own Son.
Soli Deo Gloria
Reformedish 
What about “totally necessary”? I offer this as possibility, not my fully formed position.
What does it mean for God to need something or to need to do something? If we assume that God always does what is right, and never does what is not right, and that this is the only “constraint” on his actions, then dying on the cross for us was the right thing to do. accordingly, it would not have been right for him to not die on the cross to save us.
Therefore, God deemed it necessary to save us, and therefore necessary to die as one of us on a roman execution stake.
That seems another option, but the objection from both the other positions is that it constrains the freedom of God. The freedom of God is what makes his grace, gracious. If God has a duty to save, then the implication is we have a right to be saved. But we don’t, and God does not have that duty. And yet he does out of love.
At least, that’s where I see the objection going.
“Dude, Paul, by saying that God cannot lie, you’re constraining the freedom of God.”
Why is God’s freedom of such paramount importance? Grace? perhaps we should take a closer look at what the Greek means.This is something I’ve done quite a bit of looking into.
“Grace is receiving something good which you don’t deserve;
Mercy is not receiving something bad which you do deserve”
Now, theologically speaking, that’s whatever. Let the theologians make up whatever they want. But linguistically speaking, that happens to be completely, totally, unalterably false.
Grace has nothing to do with deserts. We give grace to God. That’s the word used when you thank someone, in Ancient Greek. I’m flabbergasted when seminary student’s don’t know this.
http://www.tlg.uci.edu/lsj/#eid=116385
One could easily say without contradiction that God was “constrained” by his grace toward us.
A. Well, for thousands of years, plenty of people reading their Bible as thoughtfully as you have seem to think there is something important about the freedom of God–that he doesn’t have his hands tied about these things. There’s probably something to it.
B. You just cited the Liddell-Scott entry and then declared that linguistically grace has nothing to do with deserts, it’s the word used to thank people, and that you’re flabbergasted that seminary students don’t know about a particular usage of the term. I would simply point out that the article you posted lists various usages of the term as well as a number of words flowing from the same root. Simply scrolling through them, it seems that at least a few are related to the issue of desert, so extreme utterances something being “completely, totally, unalterably false” are a bit much.
This also seems to be the case considering that almost none of the usages here are from the LXX or the NT, in which case we have to take into account the difference involved there as well.
None of this invalidates your overall point, but it does mean there’s maybe something more to the position you’re pillorying than you’re letting on.
Okay, fair enough. I exaggerated my point without thoroughly backing it up, and that was unwise and hasty of me. Though i don’t put much stock into the “people have thought this idea for a long time, so that supports it in the face of new ideas,” type of evidence.
Nevertheless, I imagine that variants of this position would argue that either an obsession with God’s “freedom” is a misguided one, or that being constrained by his own grace nature is no more an unacceptable constraint to his freedom than any other part of his moral nature, such as the inability to lie.
Grace is given as gift unrelated to any deserts; not necessarily despite contrary deserts. Paul commands us to give grace to God, and he himself gives grace to God, in the act of thanking.
Just saying, this one might not be as open and shut as the original post implies.