Among Evangelicals, the question "Is it Biblical?" is an oft-used litmus test for whether an individual or community should accept some norm or principle. For instance, one might ask about the moral permissibility of premarital sex. The typical response in the Evangelical community would be to ask whether the claim the claim that premarital sex is morally permissible is Biblical. So what does it mean for any particular claim to be Biblical?
There are three possible relationships between claims that are pertinent here.
First is consistency. Claim p and claim q are consistent just in case it is possible for both p and q to be true together. Lots of claims are mutually consistent. The claim that Syracuse is a city in New York is consistent with the claim that Albany is the capital of New York. An example of inconsistency would be the following two claims:
James was in Syracuse on February 20, 2015, at 10:07 pm.
James was in Tokyo, Japan on February 20, 2015 at 10:07 pm.
Assuming that it is impossible for the same thing to be in two different places at the same time, these claims are mutually consistent.
Second is implication. p implies q just in case whenever p is true, q has to be true. You get this relationship a lot with definitions, as well as with reasoning about categories. Here's an example of the first.
That James is a bachelor implies that James is a male.
The second claim here follows from what it means to be a bachelor. If it's true that I'm a bachelor, then it must be true that I'm a male.
Here's another example of implication.
That Fido is a dog implies that Fido is a mammal.
Categories are often nested. The category of dog is nested in the category of mammal. So we can infer by implication that anything that is a dog must also be a mammal.
The third is equivalence. p and q are equivalent just in case whenever p is true, q must be true, and vice versa. Equivalence is basically like synonymy. Here's an example of two equivalent claims.
James went to the store today.
It is not true that James didn't go to the store today.
So we have three kinds of relations between claims. To say that some claim p is Biblical is basically to relate that claim to the set of all of the claims found in the Bible. But, what sort of relation are we talking about here? Which one of the above mentioned three is it?
Let's start with equivalence. It seems clear that p being Biblical is not a case of equivalence. It's obvious that some claim could be considered Biblical even though it's not equivalent to any particular claim found in the Bible. For example, most Evangelicals would agree that it's Biblical to go on mission trips to Haiti. However, the claim that it is good to go to mission trips to Haiti is not equivalent with any claim found in the Bible.
Having set equivalence aside, let's look at the other two relations.
Perhaps to say that p is Biblical is to say that p and the set of claims found in the Bible are related by implication. There are two possibilities. Either p implies the Biblical claims, or the Biblical claims imply p. The first would be no good. Here's why. Most Evangelicals believe that all claims found in the Bible are true. Remember, saying that p implies q is saying that whenever p is true, q must be true. The only way p would not imply q is if p were true and q were false. But since claims in the Bible are assumed to be true, p would imply q no matter what.
So, implication probably goes in the other direction. One interpretation of p being Biblical is that the set of claims in the Bible imply p. That is, p follows from the claims found in the Bible. Is this the correct way to interpret p being Biblical? Notice that this is a strong interpretation. What I mean is that it's relatively hard for a claim to be implied by Scripture. Think about it this way. In order for p to be implied by the Bible, the following has to be true:
(our set of claims found in the Bible) THEREFORE p.
How many claims is this true of? Fill in p with anything you'd like:
(set of claims in the Bible) THEREFORE one should go on mission trips to Haiti
Is that really true? Does it follow from the Bible that one should go on mission trips to Haiti? If so, how? It's not as easy as you think to make this connection.
It seems then, that to interpret p being Biblical as an implication relation would eliminate many claims that one might intuitively consider Biblical.
Now let's look at the last relation. Suppose we consider p being Biblical as a claim about consistency. So, we interpret p being Biblical as saying that p is consistent with the set of claims found in the Bible. This just means that p doesn't contradict anything that's said in the Bible. In this case, the claim that it is good to go on mission trips to Haiti would be Biblical, since that claim is consistent with all of the claims in the Bible.
Notice now that it's really easy for a claim to be Biblical if we understand this to be consistency. Lots and lots of claims are consistent with the Bible. In fact, some claims that we would normally think are not Biblical would be considered Biblical under this interpretation. For instance, the claim, "It is morally permissible to smoke crack cocaine five times a day every day for the rest of your life" is technically consistent with all of the claims found in the Bible.
So, it's not clear whether a claim being Biblical means that it is implied by the Bible or consistent with the Bible. My suspicion is that a lot of Evangelicals go back and forth between these two interpretations. If they don't want a claim to be considered Biblical, then they interpret Biblical as implication. If they want a claim to be considered Biblical, then they take the consistency interpretation.