Thanks to Kevin DeYoung for his post "Cautions for Mere Christianity."
Let me highlight two significant problems.
Atonement, But How?
The first caution to raise concerns Lewis’ view of the atonement. Lewis believed Jesus died on the cross for sin, but he didn’t think it was important to understand the particulars of what Christ accomplished on the cross.
Now before I became a Christian I was under the impression that the first thing Christians had to believe was one particular theory as to what the point of this dying was. According to that theory God wanted to punish men for having deserted and joined the Great Rebel, but Christ volunteered to be punished instead, and so God let us off. Now I admit that even this theory does not seem quite so immoral and silly as it used to; but that is not the point I want to make. What I came to see later on was that neither this theory nor an other is Christianity. The central belief is that Christ’s death has somehow put us right with God and given us a fresh start. Theories as to how it did this are another matter: A good many different theories have been held as to how it works; what all Christians are agreed on is that it does work. (57-58 [pagination varies by edition)
Later Lewis says that “Christ was killed for us” and “His death has washed out our sins” but “any theories we build up as to how Christ’s death did all this are, in my view, quite secondary” (59). This impatience of careful thinking about the atonement is bad enough, but then Lewis goes on to make clear that he rejects the understanding of the atonement evangelicals (and the Bible I would say) find most central and most glorious.
The one most people have heard is the one I mentioned before–the one about our being let off because Christ had volunteered to bear a punishment instead of us. Now on the face of it that is a very silly theory. If God was prepared to let us off, why on earth did He not do so? And what possible point could there be in punishing an innocent person? None at all that I can see, if you are thinking of punishment in the police-court sense. On the the other hand, if you think of a debt, there is plenty of point in a person who has some assets paying it on behalf of someone who has not. (59)
Pay careful attention to what Lewis says in that paragraph. He does believe in a substitutionary theory of the atonement, but he rejects penal substitution. He admits that penal substitution is not quite as silly as it once sounded, but he still does not accept it. Instead, he argues that Christ pays a debt (which is true), but not as a punishment for our sakes.
Lewis’ theology of the atonement is confusing (see for example this helpful Touchstone article), but I would argue his view is more like Christus victor or ransom to Satan than penal substitution. Aslan’s death, you may recall, was a sacrifice to the Witch and was explained rather ambiguously as “deeper magic.” This is not the place to defend the critical importance of penal substitution. My point is simply that Lewis does not teach it in Mere Christianity, and in fact undermines it.
An Early Inclusivist
The second problem with Mere Christianity is Lewis’ inclusivism. Evangelicals believe that Jesus Christ is the only way to God. Further, they believe that conscious faith in Jesus Christ is necessary for salvation (assuming we are talking about sentient beings; all Christians allow that infants and the mentally disabled may be in a different category). Lewis, by contrast, believed in what we might roughly call “anonymous Christians.” That is, people may be saved through Christ without putting explicit faith in Christ.
There are people who do not accept the full Christian doctrine about Christ but who are so strongly attracted by Him that they are His in a much deeper sense than they themselves understand. There are people in other religions who are being led by God’s secret influence to concentrate on those parts of their religion which are in agreement with Christianity, and who thus belong to Christ without knowing it. For example, a Buddhist of good will may be led to concentrate more and more on the Buddhist teaching about mercy and to leave in the background (though he might still say he believed) the Buddhist teaching on certain other points. (178)
No matter how much we may like Lewis, this is simply a profound misunderstanding of the Spirit’s mission (and a rejection of John 14:6). The work of the Holy Spirit is to bring glory to Christ by taking what is his–his teaching, the truth about his death and resurrection–and making it known. The Spirit does not work indiscriminately without the revelation of Christ in view. Arguably, the Holy Spirit’s most important work is to glorify Christ, and he does not do this apart from shining the spotlight on Christ for the elect to see and savor. Again, we see the inclusivist Lewis at the end on Narnia where Emeth, a worshiper of Tash, is accepted by Aslan for following him all along without knowing it.