"If God's moral judgement differs from ours so that our 'black' may be His 'white', we can mean nothing by calling Him good; for to say 'God is good', while asserting that His goodness is wholly other than ours, is really only to say 'God is we know not what'. And an utterly unknown quality in God cannot give us moral grounds for loving or obeying Him. If He is not (in our sense) 'good' we shall obey, if at all, only through fear -- and should be equally ready to obey an omnipotent Fiend. The doctrine of Total Depravity -- when the consequence is drawn that, since we are totally depraved, our idea of God is worth simply nothing -- may thus turn Christianity into a form of devil-worship. (p. 28-9)"
Now, C. S. Lewis is one of the best authors that Christianity has ever known, and he uses Rhetoric better than most, however, we need to ask ourselves is this right. The following is not a logical syllogism, it is simply an way of capturing what C. S. Lewis is pointing out.
(1) Total Depravity - man is so depraved that he cannot do anything good, and more importantly for this argument cannot know what good is.
(2) God is good.
(3) If (1) is right, then (2) could also mean, God is evil, and man could not know otherwise.
The point here is that Calvinism, according to Lewis claims the truth of (1) and (2). According to Lewis, if (1) and (2) are both true then so is (3). Technically we could remove (2) and simply formulate the argument as follows:
If man is Totally Depraved (as defined in 1), then man could never know whether he is serving a good or an evil God.
It should be mentioned that simply making the claim, "The Bible says that God is good (see for example, Luke 18:19)," does not help things, because, on the Calvinists view of Total Depravity, we still don't know what good is (as we are totally depraved), and if it so happened that what we mean by evil is what God means by good, then we could never know, because we base our claims of God's goodness on His word. This, I venture, is a problem for Calvinism. There may be two responses to this problem. First of all, the Calvinist may reject this interpretation of Total Depravity, or, secondly, the Calvinist may try to find a way around it, a way in which man may find out what it means when we say that God is good. I have just mentionned such a possible attempt, and I will mention, further on, another possible attempt at getting around the problem. The first point is, I think, the more important, and I will begin with it.
The Calvinist's view of Total Depravity
The main question that we need to ask is, "Does Calvinism make such a claim?" Obviously if it doesn't then Lewis's argument is a straw-man argument (in all fairness to Lewis it should be noticed that he is not necessarily attacking Calvinism, he is simply discussing the doctrine of Total Depravity, which, of course, seems to imply a problem for Calvinism).
We find a hint of what C. S. Lewis is saying about Total Depravity in Charles Hodge's commentary on Romans. In discussing the identity of the person in Romans 7:14-25 (the person says that he desires to do good, knows that the law is good and spiritual, etc.), he says, "On the other hand, those who held the doctrine of total depravity, and of the consequent inability of sinners, and who rejected the doctrine of 'common grace,' could not reconcile with these opinions the strong language here used by the apostle. (p. 240)" (Needless to say, the "those" that Hodge refers to are Calvinists.)
James White says, in a book entitled Debating Calvinism: Five Points, Two Views, by Dave Hunt & James White, "There is a fundamental incapacity in the natural man. He does not accept the things of the Spirit of God (willful rejection), for they are foolishness to him. Why are they foolishness? Because he is not a spiritual man. He cannot (not 'does not' or 'normally chooses not to') understand them. (p. 69)"
J. Gresham Machen, an author that I have found helpful on many subjects, says, in the book The Christian View of Man, that "Sin does not reside merely in the body; it does not reside merely in the feelings, or merely in the intellect, or merely in what is sometimes falsely set off from the rest of human nature under the name of the will. But it resides in all of these. The whole life of man, and not merely one part of it, is corrupt. (p. 242)" In and of itself this quote is right, but what happens is that many Calvinists then go on to say that this corruption has so affected man's intellect that man cannot know that the Bible, God's law, etc, are good.
(A note must be added, at this point, to keep in mind that Calvinism does not deny that unregenerated man are capable of doing moral acts. Charles Hodge, in his Systematic Theology, vol.2, that, "It is admitted in all the Confessions above quoted that man since the fall has not only the liberty of choice or power of self-determination, but also is able to perform moral acts, good as well as evil. He can be kind and just, and fulfill his social duties in a manner to secure the approbation of his fellow-men.(vol. 2, p. 263.)")
Charles Hodge, in his Systematic Theology, vol. 2, seeks to explain what Calvinists mean when they talk about total depravity (the sinner's inability). He says, "According to the Scriptures and to the standards of doctrine above quoted, it consists in the want of power rightly to discern spiritual things, and the consequent want of all right affections toward them. And this want of power of spiritual discernment arises from the corruption of our whole nature, by which the reason or understanding is blinded, and the taste and feelings are perverted. And as this state of mind is innate, as it is a state or condition of our nature, it lies below the will, and is beyond its power, controlling both our affections and our volitions. (p. 261)" A little further on Hodge concludes by saying that, "It is therefore the clear doctrine of the Bible that the inability of men does not consist in mere disinclination or opposition of feeling to the things of God, but that this disinclination or alienation, as the Apostle calls it, arises from the blindness of their minds...So also the whole nature of apostate man must be renewed by the Holy Ghost; then his eyes being opened to the glory of God in Christ, he will rejoice in Him with joy unspeakable and full of glory. But the illumination of the mind is indispensable to holy feelings, and is their proximate cause. (p. 263.)"
It seems, then, that the doctrine of Total Depravity, at least for some Calvinists, implies that man is unable to know that God is good, or desirable, which is more than what is required for Lewis's argument to be right. All that is necessary for Lewis's argument to work is that the Calvinist makes the claim that man cannot know what true goodness is. If we cannot know what goodness is, then we cannot know that God is good. Not only can we not know that God is good, but our very ideas about God are wrong, because our intellect is corrupted by sin. (There is alot of truth in what is being said here, the point is not to simply reject all of these points, rather, we need to see that the Calvinistic doctrine of Total Depravity seems to cause a problem. It is not the only system of interpretation that affirms that man's nature is fully corrupted, so denying Calvinism does not equate a denial of the truths of Scripture, and not being a Calvinist is not equal to not being a Christian. Contrary to a blogger at the following link, who quoted his pastor as saying, "It’s determined that you will be a Calvinist even if it’s not in this life." Dave Hunt also documents numerous quotes to the same effect in the book mentioned above, Debating Calvinism, p. 21-22.)
Yet, and this is the point of Lewis's comment, if we can know nothing about God, if all our ideas about God are wrong, then how can we know that when we say that God is good, that the term good, when applied to God, means what we think it means? We can't according to the Calvinistic view of Total Depravity, and, therefore, if the Calvinist view of Total depravity is right, then we might be worshipping an evil God.
Getting around the Problem
I already mentioned one possible way of getting around the problem: "The Bible says God is good." I also already explained why it doesn't get around the problem. I may take it on faith that when the Bible says that God is good, that he is, therefore, good, but, unless I know what that means, then I have no reason to think that God is not evil. After all, it seems like it would be entirely within the character of an evil God, to lie to his creatures and tell them that he is good. Or, to implant in them a view of goodness, and then to tell them that He is good, and let them think that what he means by goodness is the view that he planted in them, when in reality, he is totally the opposite of that implanted view. Now the problem that we see here is that if God is the exact opposite of our view of goodness, then he is in our view, evil. Man, in general has a view of evil that would include such acts as murder, rape, lying, stealing, etc. No Christian in their right minds would believe that God is capable of such acts. Yet if there is no other way around this problem, then, if Calvinism is true, then God could capable of such acts, the Bible would simply be a cover-up, lies, from an evil God to gain our trust.
Perhaps there is another way around this problem. The Calvinist could claim that God gives us, through illumination and regeneration, a true understanding of what it means to say that God is good. Now, the problem that this brings, is two-fold, first of all, it, like divine revelation, once again comes from God. If God is evil, and He lied in the Bible, then how can we expect Him to tell the truth through illumination and regeneration? We might say, well that's the thing, prior to regeneration our view of God was flawed, so we couldn't know that he was truly good, but after regeneration we are able to see God properly, so, we see what it means to say that God is good. (Aside from the first problem) This could be solved empirically. All we have to do is ask unregenerate people what goodness is, and what a good God would be like, and then compare that answer with what regenerate people say. I am going to suggest that the answers, both from the regenerate camp and the unregenerate camp will be varied, and quite similar. (I leave it to someone else to do a proper survey.) If this is true, then it would count against the Calvinist view of Total Depravity, because it would appear that the same views on the goodness of God are held by regenerated and unregenerated alike; therefore, regeneration and illumination do not provide the way out.
I would suggest that the problem is not that God is not good, or that we are left unable to know what it means to say that God is good, but that the Calvinistic view of Total Depravity is flawed. I would suggest that C. S. Lewis's argument against this view work, and that the Calvinistic view of Total Depravity must be, in part, abandoned.