Matthew 5:37

“But let your speech be, Yes, yes; No, no,

And whatever is more than these is of the evil one.”

So they are to restrict their replies to specific assertions. They are to say, ‘yes, yes' or ‘no, no'. The point here is either that people will be listening for the oath, ‘yes, I swear by --' but will rather hear another ‘yes', or that it represents the firmness with which the disciple of Christ says ‘yes' and ‘no' because they speak only the truth (compare James 5:12). It is not intended to indicate a special form of oath. The assumption is that under the Kingly Rule of God nothing but the truth will be spoken.

Indeed anything more than such a firm assertion must be seen as being the product of the Evil One (or of an evil heart). The use of tou ponerou is regularly ambiguous, compare Matt 3:39; Matthew 6:13; Matthew 13:38; 1 John 5:19. But see Matthew 13:19 where it must be translated ‘the Evil One'. Evil and the Evil One are closely connected, and the Devil is specifically linked by Jesus with falsehood. He is the ‘father of lies' and abounds in falsehood (John 8:44). Therefore here we should probably see it as signifying the Evil One. On the other hand in Matthew 5:39 it means either ‘the evil person' in the sense of one who wishes to impose himself on you, or evil itself. But here in Matthew 5:37 the Evil One has to be resisted whereas in Matthew 5:39, because it is a different kind of ‘evil', it does not have to be resisted but has to be responded to with a loving response. This brings out the wide ranges of meaning of the term.