It's an old Papal title bestowed James IV of Scotland and Henry VIII of England in the 1500s, not relinquished on the breach with Rome, and hanging round for historic reasons.
No monarch has lived up to it since that breach, and indeed acted against it - RCs banned from succession.
Though ER II did a little better in that, though, as whilst RC still banned, having an RC spouse is no longer a bar. So she did more than most.
Charles was reported pre-accession to want to remove the otiose Papal title, and replace with a new concept of Defender of Faiths. Depending on how he does it, it could be a very good thing in an increasingly diverse society.