Definitely not, for the same reason why we should not allow people to rank and rate genders, races, or cultures. With most of what we have here on OpiWiki, there is at least some degree of these two things: objectivity and choice.
By objectivity, I mean simply that users can theoretically agree on a set of objective criteria that allows them to debate the merits (or lack thereof) of any given...thing...with legitimate points ascribing to that criteria. Let's take, for example, movie directors. There is at least some (only some, but still some) degree of objectivity that goes into debating whether or not a director is good. Nearly all users can say, "this is good, and this person does it well," while others say "yes, this is good, but this person does not do it well." The "this is good", the common ground, is what leads to good debate, and is what can easily exist in ranking of most items.
But when it comes to religion, agreeing on any sort of baseline is impossible. That's because religion itself is, more or less, a baseline, a way of thinking and a way of knowing. You cannot rank thought processes or ways of life, because it's very simple: you either are part of that way of life or you aren't. Either way, there's no possible way you could give a fair judgment or make anything resembling a strong argument to support your views.
Besides, simply put, no-one has the moral authority to judge a different way of life. Imagine if we had a "black people" ranking. Absolutely unthinkable. Who is in a position to judge something like that. Maybe if you're black and know the culture, you might have something constructive to say. But we have no way of verifying anything like that online, and the likelihood is that a ranking of that sort would do nothing but attract the Neo-Nazis and Klu Klux Klan roaming the internet.
A similar thing would happen with any ranking of any religion. It would likely attract only the people who prefer to hate on it, and would therefore result in what would likely be a very low score.
The effects on the site are threefold.
First, it will offend people. Now, I know that people simply being "offended" is not a reason to block out a ranking, but we don't want OpiWiki to develop a reputation as a site that allows for, tolerates, or even accommodates religious discrimination. When people see this score and take it to heart (as they will, and as I would if I found my religion in the rankings), they will see it not as a reflection of the limited user-base voting on it, but as a reflection of the site itself.
And they may not be entirely wrong. Second, allowing for any type of cultural ranking, including and especially religion, is an invitation to those who would rather discriminate against religions (imagine what a nightmare a ranking on Islam would be). It's like putting up a flag saying "here's a place where you can safely express your discriminatory views in what is presented as an objective and statistical manner." This flag could entirely change the OpiWiki community that we strive for; it could shift the base away from an intelligent, discussion-based demographic to a base who simply loves to hate, to give "the other" low "scores" in a seemingly objective manner, simply for the sake of it.
Third, it makes it far more difficult for us to draw lines...anywhere. Opening up the door to religious rankings puts us on shaky ground, mostly because you can't really separate religion from race and culture. If someone says they want to put up a ranking for, I dunno, "Nigerian Culture", and we say, "oh no, you can't do that," and then they point to Christianity and say, "but they got to rank a way of life, why can't I?" We could throw up some gates about specifically mentioning race, but then what to make of Judaism, which is as much a race as it is a religion? And as soon as we open up that door, allowing users to create ranking for entire groups of people, what's to prevent someone from making a ranking for "gays" or "transgenders"? Yes, we could throw up more walls, but these walls would seem arbitrary due to the fact that we've already permitted something like religion, which is oftentimes not a choice and which oftentimes does govern much of a person's way of life and way of thinking. It's like we've opened the flood gates, and are trying to build beaver dams to hold it all back. The very worse case scenario? OpiWiki could be used as a tool for cyberbullying, with users adding ranking for individual people for others to rank down and belittle.
Given, the cyber bullying is an unlikely worse case scenario. But considering that there are so many large negative effects and larger potential negative effects, and considering that little to no "meaningful, serious, or prestigious" ranking or conversation can come out of religious ranking, let's just keep those flood gates closed.