Martin Lloyd-Jones, the great twentieth century Welsh preacher, wrote the following: "C. S. Lewis had a defective view of salvation and was an opponent of the substitutionary and penal view of the atonement." (Christianity Today, 1963 quoted in the article "Mere Atonement: C. S. Lewis & the Multiple Angles of Redemption" by Ariel James Vanderhorst, Touchstone, April 2009)
The point of Vanderhorst's article is that Lewis's view of the atonement is difficult to discern. One can imagine what Lewis thought about the atonement if one reads the whole Lewis corpus. The article "Mere Atonement" beautifully summarizes the various expressions of Lewis's view(s).
This morning I read the following passage in Lewis's Miracles: A Preliminary Study that helps me see how orthodox Lewis really was.
"Because the higher can descend into the lower He who from all eternity has been incessantly plunging Himself in the blessed death of self-surrender to the Father can also most fully descend into the horrible and (for us) involuntary death of the body. Because Vicariousness is the very idiom of the reality He has created, His death can become ours."
Perhaps that paragraph is difficult to understand out of context. If so, I urge you to read the whole chapter titled "The Grand Miracle." Indeed, I urge you to read the whole book.