He also believed in the Real Presence.
Unusual for a man raised in Protestant Belfast . . . !
But you have to remember what his day job was -- professor of medieval and Renaissance literature. As his fellow countryman (and candidate for sainthood) John Cardinal Newman said, "to be deep in history is to become Catholic."
A lot of my closest evangelical friends (well, former closest friends - they dropped me like a hot potato when I converted) read nothing more of Lewis than Mere Christianity and the Chronicles of Narnia. But there’s so much more to read than those two, so it’s likely that they haven’t encountered his sacramental views.
“to be deep in history is to become Catholic” -> Amen, sister! I’ve also seen it quoted as this: “To be deep in history is to cease to be Protestant.” But the effect is the same nonetheless.