We do not know the religious affiliation of Dr. Cecilia Reyes. Maybe she is a Seventh-Day Adventist.
In considering major international religious denominations and religious groups for which we had yet to identify any representative super-heroes, we noticed that the Seventh-Day Adventist Church was completely un-represented. Desmond Doss has been portrayed in comic books a few times. Desmond Doss was a devout Seventh-Day Adventist who was a conscientious objector but became a war hero as a medic.
But comic book adaptations of a real-life hero aren't quite the same thing as having a traditional comic book character with super-powers, a costume, etc. So we tried to think about what super-heroes might be Seventh-Day Adventists, and Cecilia Reyes came to mind.
Cecilia Reyes is a doctor, and this calling is rather central to her identity. In fact, she never really wanted to be a super-hero. She kind of fell into it when she crossed paths with the X-Men, and it wasn't long before she left her life as a mutant super-hero to once again become a full-time doctor. Cecilia Reyes was the only member of the X-Men who was a true doctor of medicine (rather than a scientist who knew some things about medical treatment). Cecilia Reyes never even adopted a super-hero codename (although she did wear spandex).
Among Seventh-Day Adventists, being a doctor is one of the most common professions. Seventh-Day Adventists have a higher proportion of their members working in the medical profession than any other religious group. Seventh-Day Adventists are famous for their highly regarded hospitals all over the world. Medical practice is so significant to Seventh-Day Adventist life that the church is sometimes regarded as a quaint "hospital sect." The fact that Cecilia Reyes is a devoted doctor makes it significantly more likely that she is a Seventh-Day Adventist.
The fact that Cecilia Reyes left her life as a super-hero is also significant. Seventh-Day Adventists have a strong history of conscientious objection to war and combat. Real-life Seventh-Day Adventist war hero Desmond Doss famously refused to carry a weapon. Similarly, Cecilia Reyes is very much uninterested in combat. In fact, it is interesting to note that her super-power was almost purely used defensively: she could project a force field around herself and others. Eventually Cecilia Reyes learned to project her force field into simple offensive weapons (such as a blade), but she rarely ever did this.
Cecilia Reyes is Puerto Rican. According to the Adventist yearbook statistical reports, there were 260 Seventh-Day Adventist congregations in 2006, with 64,256 adherents, or about 1.64% of the total population.
So, yes, if Cecilia Reyes is classified as a Seventh-Day Adventist, there is probably a bit of "affirmative action" at work. We know of no actual published comics or other media that actually identifies Cecilia Reyes as a Seventh-Day Adventist, nor are we aware of anything in comics that that strongly hints she is an Adventist. But this is a plausible and possible identification, as explained below. But, to be perfectly clear, this possible identification came about largely because we felt sorry for Seventh-Day Adventists. There are more practicing Seventh-Day Adventists in the world than there are observant Jews, yet there are hundreds of Jewish comic book heroes and no SDA super-heroes.
Why is this? Why are there so few (essentially zero) Seventh-Day Adventist super-heroes and relative to vastly greater numbers of super-heroes from other, numerically smaller religious traditions? Certainly one reason is the fact that Jewish writers and artists were so prominent among foundational comic book creators, while Seventh-Day Adventists generally frown on any science fiction, including comics. Another reason is that the Jewish tradition is so ancient (thousands of years old), whereas Seventh-Day Adventism is relatively recent (dating back to 1863). Fiction writers tend to be more comfortable writing about older versus newer religious cultures. A comic book writer who may be extremely hesitant or fearful about writing about a Seventh-Day Adventist (whose religious tradition is less 150 years old) would have no reservations about writing about characters who worship ancient Egyptian or Greco-Roman classical pantheons from thousands of years ago.
Should there be more Seventh-Day Adventist comic book characters (or any SDA comic book characters)? Yes, of course! Seventh-Day Adventists represent a highly distinctive, cohesive, successful sub-culture. Seventh-Day Adventists have their own unique beliefs, practices, traditions, and culture. Simply identifying a comic book super-hero as recently re-committed Seventh-Day Adventist or a convert to Seventh-Day Adventism would open up an entire world of fresh possibilities, story ideas, nuances, etc., that would breathe life into the character. (Of course, the same can be said for essentially any distinct and cohesive yet fictionally under-represented sub-culture.)
Take "C-List", limbo-dwelling, often-forgotten character like Cecilia Reyes . . . What is really unique about her? She's a medical doctor, okay, but so is Thor in his Donald Blake identity, so is Dr. Mid-Nite, and so are a hundred other comic book characters. Hispanic (or black)? We could name hundreds of super-heroes who fit that description. There are dozens of Puerto Rican characters in Marvel comics and hundreds in American pop culture. The personal force field? How many times has that been done? Are Cecilia Reyes' super-powers really different from the powers of Skids, or Unus the Untouchable, or Invisible Woman or any number of other force field producers? But have Cecilia Reyes re-commit to the Seventh-Day Adventist Church she was brought up in and that inspired her to go into medicine, have this really be a part of her character, and you've instantly made her into a character who is potentially more interesting and surprising than a hero from the most elaborately imagined alien culture.
Just as Korean-Americans are and are not similar to the rest of American culture, just as lawyers are and are not just like everybody else, just as Jews, LGBTs, Catholics, Vegans, cops, grifters, carnies, Gypsies, deaf people, Mensans, Alaskans, cancer survivors and Baha'is are and are not like all other Americans, Seventh-Day Adventists are both similar to and very different from mainstream American culture. As such, they represent rich and untapped literary potential.