I took a seminary level course in the life of C. S. Lewis. He’s so popular, everyone from Baptists to Eastern Orthodox or Roman Catholic tries to claim him. So much for “mere Christianity!”
Lewis’ brother, “Warney” a bachelor who lived with C.S. (”Jack” as C.S. Lewis was known to his friends) was a horrendous binge alcoholic. He would literally disappear for weeks at a time, on a bender, and not remember what happened. Very very bad. He was even drunk when Jack died, and that state may have contributed to his ignoring C.S. Lewis’s rapid decline....(and he blamed himself for it...dying not too long afterward, staying drunk most of the time). Anyway, one time after a bender on a vacation Warney was picked up out of the gutter (literally) by some kind nuns at a local Roman Catholic hospital. They nursed him back to health and were so loving and kind, Warney was ready to convert. Jack though warned him in a letter in the most stern terms not to do it, and Warney didn’t.
We must remember, C. S. Lewis was actually Northern Irish in his upbringing, and Welsh in his ancestry—his grandfather a preacher...no background that makes one sympathetic to Roman Catholicism. He was a high Anglican, even fairly Anglo-Catholic (I’m sure they will claim him... believing in a form of purgatory for example—but not as dogma), but that didn’t make him, if you’ll excuse the expression (which Lewis used), a papist.
And poor Warnie! He was a scholar in his own right, he did try to beat the bottle, but it eventually beat him. There wasn't the support and treatment available in those days that could have saved him.
I think it's actually Lewis's adherence on most occasions to the idea of "mere Christianity" that makes it possible for so many folks to claim him! (and that's not necessarily a bad thing). He says in the foreword to Mere Christianity that the concept is like an entryway or hall to the various Christian denominations, and that you can't stay in the hall. But of course that's where the book is -- basic tenets that almost all Christians can agree with. In the words of a salesman, "Get 'em in the door!"