There are still individuals who have attempted–often in times of some great desperation–to worship mythical gods told of in only the oldest of songs. Those who push their faith far enough are indeed able to cast holy spells. However, the moment anyone achieves this and becomes a Cleric, they go irrevocably insane. Not “slightly-touched-in-the-head” insane, but full-out balls-to-the-wall insane. These Clerics are still able to cast spells, but it’s a largely wasted talent, as they have no ability to apply anything intentionally. They babble nonsense and wander for the rest of their lives, and more than a few die of starvation/dehydration, too out of their minds to realized they need to eat and drink. Most cease to respond to their own names. Unsurprisingly, these folks are relatively rare, though they pop up often enough for most common people to have at least heard of them, if not to have interacted with them personally.
Nobody knows what the connection is between being able to cast holy magic and losing one’s mind. Several theories have been proposed, but there is no agreed-upon answer and very little by way of active research on the cause.