The fictional character of Supergirl is Superman's cousin Kara Zor-El. Unlike Superman (a.k.a. "Clark Kent", whose birth name is "Kal-El"), Supergirl was raised on Krypton. She was a teenager when her parents put her into suspended animation in a space capsule that saved her life by bringing her to Earth away from the doomed planet Krypton.
Krypton had a scientifically advanced society with a rich religious culture. Kara Zor-El was raised in the orthodox Kryptonian religion. Kara Zor-El has never been portrayed as being particularly religious, and there is no indication that she has actively practiced Kryptonian religion since coming to Earth in the sense of participating in worship services and rituals. But nor is she is known to have rejected the religious teachings she was raised with in any way.
Below: Supergirl invokes the name of the Kryptonian god Rao. Supergirl was actually raised for many years on Krypton and has vivid memories of her childhood there. In fact, excluding the time she was in suspended anmation travelling to Earth, Supergirl spent most of her life thus far on Krypton, not Earth. Thus, it is not surprising that her religious identity was formed largely on Krypton. In a way similar to Superman of Silver Age and pre-Crisis comics, Supergirl routinely invokes the name of Rao in occasions where Americans might refer to "God." This is the case below, with Supergirl's exclamation to the annoying bounty hunter Lobo: "Rao above, what is wrong with you?" [Source: The Brave and the Bold #4, published by DC Comics (August 2007), page 12; written by Mark Wait, pencilled by George Perez, inked by Bob Wiacek.]
From: Trevor Barrie
Date: Sat, Oct 19 1996 12:00 am
...the pre-Crisis Superman was definitely not Christian. He worshipped the Kryptonian sun god. Post-crisis Supes has never been explicitly placed in any religion, but considering his upbringing, he's probably a Christian of some flavour.
From: Matthew Daly
Date: Mon, Oct 21 1996 12:00 am
Great Rao! I thought that was just Supergirl's faith.
From: "Superheroes/villains and their religions" forum discussion, started 16 March 2006 on "Animation Insider" website (http://www.animationinsider.net/forums/archive/index.php?t-17835.html; viewed 28 June 2008):
03-16-2006, 05:16 AM
Someone pointed this out at another forum. I found it to be quite amusing that someone would actually have enough time on their hands to ponder about this.
09-19-2006, 09:25 AM
Does this include animated DC continuity or not? ...in animated Smallville, Supergirl mentions a Pastor Ross ("Unity"), and a church is shown a lot, indicating that he [Superman] does (or did) attend church in Smallville.
From: "Superhero Religious Views?" forum discussion, started 9 June 2007 on Newsarama website (http://forum.newsarama.com/archive/index.php/t-116001.html; viewed 13 July 2007):
06-10-2007, 08:42 PM
Let's see this from the hero's point of view. Superman is always saying "Rao", so he might well be a worshipper of the Kryptonian gods. Supergirl and Power Girl were raised in a Kryptonian lifestyle, so they would certainly be worshippers of the Kryptonian gods...
06-13-2007, 01:43 AM
...Wonder Woman, Troia, and Wonder Girl obviously worship the Greek gods. So does Aquaman (primarily Poseidon). I imagine that Supergirl does too due to her time with the Amazons...