You consider his generation (that of my grandfather, WWI veteran) and how he was raised (an Northern Irish Protestant) and Lewis was remarkably free of religious bigotry, specifically anti-Roman Catholic bigotry. He was good friends with J.R.R. Tolkien after all, among other Catholics, and like good Anglicans, even (or especially) theological conservative ones, had a big-tent idea of “mere Christianity” as you said.
He, like most Anglican and Protestant types today, even evangelicals, freely admit its very possible to have a personal relationship with Jesus, and yet remain a Roman Catholic. While this may not sound like something big, just a generation ago this could not be said, evangelical types admitting RC’s can be born-again....and at that time neither would many RC’s reciprocate the idea of non-Catholics going to Heaven.... Times have definitely changed.
I hope this is a true ecumenicism, and not just a product of post-modern relativism...”if it’s good for you...”
For his generation, though, you are absolutely right that he was far more perceptive and tolerant than most. He certainly recognized and rejected the extremes of Orange bigotry. Since you studied Lewis in depth, you probably read Pilgrim's Regress, and the prejudices of the church of his childhood don't come off very well there.
My maternal grandfather, also a WWI veteran, was a rock-ribbed Scotch Presbyterian of the old school, and anti-Catholicism ran pretty deep. If he were still alive, he would be shocked and revolted by our "poping". Even my mother was viscerally horrified -- she got over it when we finally got her to Mass and she realized that we weren't sacrificing babies or anything. "It's just like our service!" she exclaimed in some surprise (she became a Piskie when she married my dad). Well duh, mom.
And there's no question that things have changed a great deal from the time of our grandparents -- or even from my childhood when people told the most outrageous stories about Catholics. I think creeping secularism has a lot to do with it -- Christians stick together when they're persecuted. When almost everybody was at least nominally Christian, it was easier to quarrel over distinctions between groups, because the core beliefs were never challenged.