I’ve never bought that. There would be a variety of opinions on the matter and many different faiths from Earth and the various Federation planets, just as there are now. Humans would likely have converted to some of the alien religions and there would be blending of Earth religions with Vulcan, Klingon, etc. to form new religions. Phlox from Enterprise had attended services in a church while he was on Earth and the Discovery crew discussed their various religious beliefs or lack thereof. Klingons have a belief in an afterlife, prayers, and a monastery that B’Elanna Torres was sent to for education as a child. Spock’s half brother had a belief in a mythological paradise. Vulcans offer prayers at a temple for the safe return of family members and have complex rituals involving monks and priestesses and the katra. Romulans have an order of warrior nuns and a number of gods, along with meditation rituals that resemble those of Vulcans. The Bajorans on Deep Space Nine are religious.