The core criteria for writing answers on the Stack Exchange model, not only on this site but across the entire network, is one of usefulness; this can be reflected simply by hovering over the vote buttons on the side of every post:

Usefulness, unfortunately, is highly subjective. Unlike our progenitor Stack Overflow, where something merely needs to be correct to be useful (and this is often easily verifiable), Christianity is a multi-faceted religion, often with multiple equally viable viewpoints. And more often than not, these viewpoints require significant study and knowledge to either confirm or deny.
This issue has been around pretty much since Stack Exchange started expanding out of Information Technology and into a more general Q&A network. Robert Cartaino covers it in his blog post "Good Subjective, Bad Subjective," which is and remains highly recommended reading for anybody posting answers to this site.
However valuable the above blog post is, it only covers the general case of subjectivity across the network. In this post, I will attempt to focus on what makes an answer "useful" to Christianity.SE in particular.
Being correct is not the same as being useful.
One of the qualities of the internet, especially for a site like this, is the fact that we're all pretty much equal (some would call that an advantage). You could be Pope Benedict XVI or John MacArthur, or you could just be Joe Sixpack from Milwaukee, and we really couldn't tell the difference.
One effect of that is, when proclaiming an answer, we the community (and especially future users) have no way of knowing if the poster is speaking from experience, knowledge, or just mere opinion. Even if Joe Sixpack is an expert on all things Milwaukee, that doesn't give him the authority or expertise to write a theology textbook.
One key factor of Robert's above blog post is the concept of the "Back It Up! Principle", which he describes thus:
Back It Up! means that your answers must be based on either:
- Something that happened to you personally
- Something you can back up with a reference
While I have seen questions that can be (and have been) effectively answered by no more than the above criteria, I feel that there is one key factor missing in this description, at least as it pertains to our site:
- Something you have the authority to say.
The simple fact is, anybody can claim anything as a fact. Obviously, that doesn't necessarily mean it is a fact; no matter how many times I say the sky is green, it won't change the fact that the sky actually is not green (at least not where I'm sitting). But whereas the color of the sky is easily verifiable by looking out the window, confirming the veracity of a lot of Christian information is not that easy.
As a lot of questions on this site involve salvation, sin, events that happened over two thousand years ago, and so on, authority is a major concern. Presumably, none of us were around to personally sit at Jesus' feet and hear His teaching. In lieu of any personal authority, if we're going to answer such questions we need to establish upon what (or whose) authority we do speak.
And as mentioned above, since there's no real method to prove or disprove anybody's identity, the poster's own authority is negligible; all posts should be written as if by an average person on the street. If any post makes claims that this hypothetical average person does not have the authority to make, then Back It Up!. This means that even if N. T. Wright were to create an account, he would still be expected to provide evidence to support any of his claims.
Unless easily verifiable, making a claim without clear authority is never useful, regardless of whether or not it's right. What makes such a claim useful is either:
- referring to reputable research with verifiable methodology
- laying out the evidences in full
If the poster is not able to produce proofs for any claims made in the post, those claims are not persuasive. As Paul tells us, "test everything; hold fast what is good."
(1st Thessalonians 5:21 ESV) Voting is our first line of defense.
What, then, is useful evidence?
There is no clear response to this. Some Christians will only accept evidence from Scripture. Others will only accept evidence from their own church's doctrine. Still others will only accept evidence from reason and observation.
In cases where the question explicitly requests particular evidences, or from the perspective of a particular denomination or tradition, usefulness is fairly easy to judge: Answers lacking the requested evidences are not useful.
In summary
Regardless of the methodology used, or the evidences cited, the usefulness of any claim (and thus answer) depends almost entirely on making clear the following two points:
- It is not mere opinion
- It comes from an authoritative source