Jesus seems to have forbidden swearing, insisting that our “yes” should be a yes, and our “no” a no and that everything else is “from the evil one” (Matthew 5:34-37). A strong reading of this would take Jesus to be forbidding all oaths. By and large, the Christian tradition has not taken that to be the correct reading. In the US, many people who accept the Bible, including presumably this text in Matthew, swear in court on the Bible.
A plausible reading is that Jesus is engaging in hyperbole to command an integrity such that there be no need for oaths. For Christians, the same norms of integrity apply to simple assertions as to sworn depositions. I will assume that this is the correct reading.
Suppose one thinks, contrary to the main line of the Christian tradition, that it is sometimes permissible to lie. Then one has to think that it is also permissible to lie under oath in precisely the same circumstances in which it would be permissible to lie without oath. But this is an implausible consequence.
Let’s say that it’s permissible to lie to save an innocent life—that’s one of the most given criteria. Then, we have the consequence that it is permissible to swear to a false alibi for someone whom you know to be innocent in a capital case if you foresee that otherwise he will be convicted. This is already implausible. But it also means that, implausibly, even in a good and free society one might have good reason to keep one’s moral views secret. For if it were known that according to one’s moral views it would be permissible to provide a false alibi when one took oneself to know that the accused is innocent in a capital case, then one’s true alibi would be of little worth in such cases, too.
Second, whatever criterion one has is going to admit of borderline cases. For instance, in the case of “saving an innocent life”, at one end, there is the case where it is certain that the person will live if and only if one lies. Near the other end, the probability of survival is slightly bigger if one lies. It is implausible that one can permissibly lie just to give someone the slightest increase in probability of survival. If that were so, then it would be permissible to swear to a false alibi (assuming, as per my reading of what Jesus said, that swearing falsely is permissible whenever false assertion is) even when it is very likely that the innocent accused in the capital case will be let off, as long as it slightly increases the innocent’s chance of survival, and that can’t be right. And somewhere in between there will be borderline cases.
But now consider a borderline case of the permissibility of lying. If it is only borderline whether it is permissible to engage in a simple lie, to swear falsely in such a case would be simply wrong, not just borderline wrong. But this violates Jesus’s principle that the norms regarding simple lies are the same as the norms regarding false oaths.
It seems to me that the best reading of the situation is that:
Lying under oath is always wrong.
And so, lying is always wrong.