I'm a Christian, albiet not a deeply spiritual one. I think these guys take things too far and are more a group of rabble rousers than anything.
What's more it seems like they don't even take their message seriously, and are acting like a group of bullies. Going after comic-con on these grounds seems a bit weak, even as they explain it. The logic just doesn't follow since I've never seen anyone pray to comic book characters that I can think of.
I suppose if your a super hard core Christian and think that you should ONLY be reading The Bible, well I can see how they could argue that comics take away from that, but then again so do most things (TV, Books, etc...).
If I agreed with the message (which I do not), I would probably have chosen to go after "Star Wars" to be honest. The reason being that people have allegedly been forming religions based off of it. I read a while ago that enough people on the Canadian census (I think that's what it was) answered "Jedi" when queried about what their religion was. Supposedly after this a few people took the steps to actually get Jedism (various versions) recognized as a valid religion. How many people are actually signed up for it I have no idea... but all jokes aside (since this strikes me as being a stunt) you could actually use the logic to claim the corruption of Star Wars since people are actually claiming to be practitioners of a fictional school of mysticism.
Of course they won't go after these guys because chances are there aren't enough of them gathered in one place (or acting seriously enough) to picket, and there just isn't the kind of media coverage that they get from harassing a big time comic convention.
The overall point being that I think this is all about the attention.
For my part despite being a Christian I pretty much let people do their own thing. I don't believe in trying to convert people. I might think it's ridiculous, but honestly if someone DID want to worship Superman or whatever, I'd pretty much take a 'more power to you' attitude.
Despite what the Bible says at points, I believe god is a fundementally good being, and I don't think that such things would go beyond his abillity to forgive. Just as I don't think people who write fiction that might portray him in a negative light for the purpose of making a point or whatever are going to be damned (ie Spawn, Preacher, Neon Genesis Evangelion, Incarnations Of Immortality, and stuff like that... it's all fantasy and understood to be fantasy). I could be wrong, but that is my gut feeling on things and I guess it comes down to faith (this is what I think when I think about such things).
So basically what I'm getting down to is that I don't think this paticular "Church" is right in what their doing. They seem to be out to get attention and nothing more. I cannot see much good that could be expected to come from this paticular rally given that I can't think of ever having seen the behavior they are protesting. Even if there are some people out there who follow say "Superman" as a religion in a literal sense, I can't see them being numerous enough to warrent a full on protest at Comic-Con.