C.S. Lewis was by no means anti-Roman Catholic. He had a number of very close friends who were Catholic, not least of whom was J.R.R. Tolkien. Despite his deep respect and love for Catholics, Lewis chose to be an Anglican rather than a Roman Catholic.
Melissa Cain Travis has written a very interesting post on why Lewis was not a Roman Catholic. It is worth a read.
I’ve done quite a bit of biographical reading on Lewis, and one of the things I love about him is how well he related to Christians of other traditions. He truly lived out his “mere Christianity” philosophy, which so beautifully reflects Christ’s heart for the universal church. It is a philosophy that I strive to emulate both professionally and on a personal level. As a champion for the mere Christianity ethos, Lewis very rarely wrote publicly about why his chosen “room” of Christendom was Anglicanism, or why he chose Protestantism over Roman Catholicism. However, he carried out private conversations and correspondence with his academic colleagues, acquaintances, and friends who were Roman Catholic laypersons or clergy about why he was so firmly Protestant. Importantly, he did so without being argumentative and with admirable graciousness.
You can find the full post here.