1- an individual child's talents and interests being nurtured in a specialist environment
But to get in the school to be nurtured you have to have done several years music lessons.
I'm Catholic, as are my family. We go to church. We're also sporty. My kids have done sport since they were able because that's what our family does. Sport on saturday, church on sunday. Because they've been doing sport form an early age, they're ahead of peers who only picked it up when they were old enough to express an interest, or join a school club.
Similarly with music. We're not a musical family. I wouldn't know where to start, or the best age to start. By the time mine got to Yr 4, and we realised about the musical selection, and they were old enough to start saying, hey mum, friend x plays the clarinet, can I try?, friend x had already had 2-3 years of lessons, had family at home to help practice, and her own musical instrument. Whatever my childs natural ability, chances are we couldn't have afforded instrument hire or the lessons anyway, even if someone had picked up on that ability.
It's as much about your family background as it is about ability. Dc's peers have had music lessons and SATS tutoring from an early age, specifically with the intention of a music specialist or grammar school.
Personally I don't think you'll ever get truly non selective schools. Rich people will simply buy houses next to the good school, and those not well off will get what's left. I just think faith is as good an arbitrary reason as any, and at least the church financially contributes. Selection on faith and/or ability at least gives poorer people a chance.
As several posters have said, church schools these days are inclusive, don't indoctrinate, and follow national curriculum with regard to RE. Most faith schools take children not of that faith, or any faith.