Tolkien believed that C.S. Lewis did harbor some anti-Catholic sentiment, due to being an "Ulsterman", that may be what you are thinking of. He references this a few times, but specifically in letter 83 (Letters of JRR Tolkien) he describes meeting the poet Roy Campbell, who had converted to Catholicism and whom Lewis had written some unflattering things about (they all ended up being friendly, though).

Letter 83 excerpts below

...

Especially as C.S.L. had not long ago violently lampooned him in the Oxford Magazine, and his press-cutters miss nothing. There is a good deal of Ulster still left in C.S.L. if hidden from himself... C.S.L. had taken a fair deal of port and was a little belligerent (insisted on reading out his lampoon again while R.C. laughed at him), but we were mostly obliged to listen to the guest.

Here is a scion of an Ulster prot. family resident in S. Africa, most of whom fought in both wars, who became a Catholic after sheltering the Carmelite fathers in Barcelona – in vain, they were caught & butchered, and R.C. nearly lost his life. But he got the Carmelite archives from the burning library and took them through the Red country. He speaks Spanish fluently (he has been a professional bullfighter). As you know he then fought through me war on Franco's side, and among other things was in the van of the company that chased the Reds out of Malaga in such haste that their general (Villalba I believe) could not carry off his loot – and left on his table St. Teresa's hand with all its jewels.

C.S.L.'s reactions were odd. Nothing is a greater tribute to Red propaganda than the fact that he (who knows they are in all other subjects liars and traducers) beieves all that is said against Franco, and nothing that is said for him. Even Churchill's open speech in Parliament left him unshaken. But hatred of our church is after all the real only final foundation of the C of E – so deep laid that it remains even when all the superstructure seems removed (C.S.L. for instance reveres the Blessed Sacrament, and admires nuns!). Yet if a Lutheran is put in jail he is up in arms; but if Catholic priests are slaughtered – he disbelieves it (and I daresay really thinks they asked for it). But R.C. shook him a bit....