1. Self-help book idea: A Year Without a Lie.
2. A number of vices in some sense require a willingness to lie. One can't really commit adultery without being willing to lie. Here the "can't" is of a restricted prudential rationality. For instance, in many cases one can't cheat on one's taxes without either lying (writing down a falsehood on a tax return) or at least being willing to lie (destroying records and not filing, while planning to lie about the destruction should one get an audit). It would probably be hard to be an unscrupulous politician without a willingness to lie.
An absolute unwillingness to lie will keep one from a number of vices. What about a merely prima facie unwillingness? An unwillingness to lie unless by lying one prevents a great evil? Well, psychologically speaking, such an unwillingness is likely to be weaker. Moreover, such a conditional unwillingness is unlikely to keep one from lying to one's spouse if one commits adultery since one is likely to think that by so lying one is preventing a great evil—especially if one has children. Similarly, it may not keep one from lying to a tax auditor, since a tax fraud conviction may result in a great evil to oneself and one's family. So there is reason to adopt an absolute unwillingness to lie in order to keep oneself from other vices.
3. There is a value in being the sort of person who can be trusted no matter what. If one is known to conscientiously follow the rule not to lie unless by lying one prevents a great evil, then before relying on one's testimony, others may have to try to figure out whether one might not think one is preventing a great evil, and this will lower the value of one's testimony.
Then there will be circumstances in which one's testimony is of no weight, and yet it is of vital importance that one's testimony have weight. Suppose, for instance, that I believe that my friend is innocent of murder. I testify to the court: "He spent the evening with me, talking about Spinoza." Suppose all the other evidence is against my friend. If I am the sort of person who lies to prevent great evils, then I am the sort of person who would provide a friend whom I believe to be innocent with a false alibi under those circumstances, because I would thereby be preventing the great evil of his being falsely convicted—indeed, his life might be stake. Therefore, for my testimony to carry the weight that it is desparately important for it to carry, I have to be believed to be the sort of person who wouldn't lie even to save one's friend from what I believe to be an unjust murder conviction.
Or, for a more common case, consider an innocent wife who is asked by a jealous husband whether she was faithful to him. If she is known to follow the rule of not lying unless lying prevents a great evil, her testimony to her innocence is of little worth, because quite possibly a great evil would be prevented by her lying if she were unfaithful. But it is crucially important that her testimony be believed, and this requires that she be known to follow the rule of not lying simpliciter.
Now, granted, such cases may be rare. But I think they are no rarer than the cases where lying is needed to prevent a great evil, and in fact they are more common. Here's a handwaving argument for this. Both kinds of cases are a species of this situation: It prevents a great evil if one's interlocutor comes to believe that p. But it seems unlikely that most species of this situation are such that in fact p is false. There are two views on the matter that one might hold. One might think that this situation occurs just as often with p false as with p true. But a more correct view of this is that true belief is somewhat more likely to be beneficial to society than false belief. If so, then a majority (though perhaps a modest one) of cases of this situation are ones where in fact p is true. (To get the desired conclusion from this, one has to either assume that that the speaker knows whether p, or, more weakly, that the speaker is more likely to be right about p than to be wrong about p.)
So it is at least as important, and likely more important, that one be believed no matter what than that one be able to lie to prevent great evils.
Therefore, it is a good thing to be such as to be believed to be unwilling to lie, no matter what. But the best way to ensure that one is believed to have that sort of character is to have that sort of character. Moreover, if one fails to have that sort of character, but pretends to do so, such a constant pretence is likely to be harmful to one's character. And it is unlikely that someone who constantly pretends to have a character other than she does is going to be the sort of person whom people believe no matter what. Therefore, there is good reason—even good consequentialist reason—to adopt an absolute unwillingness to lie.
4. The above points apply particularly strongly to Christians because it is particularly crucial that people believe our testimony about Christ. We believe, after all, that whether people have faith in Christ affects their eternal well-being. Thus, whenever we speak with someone about Christ, this is a situation where a great evil and a great good are at stake in our testimony being trusted.
Suppose that I thought it was acceptable to lie to prevent great evils. Then if I have an atheist friend who trusted me, I might well conclude that the right thing for me to do is to testify to having seen some miracle that I haven't in fact seen. (I might try to limit the extent of the lie, for instance by choosing some miracle that I read about and that I believe happened, and lying only about whether I myself had witnessed it.) But of course if I am known to be the sort of person who lies to prevent great evils, my atheist friend would have no reason to trust me. On the other hand, if I am known to be the sort of person who would not lie even to save someone from eternal damnation (not that one's words literally do that—but they may in some way contribute, because God's grace works through them), then if I tell my atheist friend that I have seen something miraculous (not that any of the miracles that I've seen are going to be that convincing to the atheist, since they're all miracles of the beauty of nature, and miracles of moral transformation in myself—I am a sinner in a bad way, but you should just think what I'd have been like without Christ!), my friend may very well believe me. And likewise if I testify to less overtly miraculous things.
The above also gives the Christian reason to believe responsibly—this may or may not imply evidentialism.