I love how every few years, someone in this industry tries to reinvent the wheel. I can't quote you the case law, but this has been tried. The end result was because you work under a union contract, and your authorized union bargaining agent waived your right to have Sunday off via the contract scheduling and seniority sections, you don't have a right to refuse to work Sundays on religious grounds. Basically, the arbitrator determined that you have waived the right to an accommodation by accepting employment under the terms of the contract.

More interestingly, even SkyWest flies on Sunday, and I think everyone would agree they are the holiest of the airlines, and also non union.

Chick-fil-A loses some business, but they save a lot more they'd they lose by closing shop and not paying anyone on that day. It's not as much about the holiness as it is about the money. Sunday just happens to also be a low traffic day for fast food.