1. Incorrect Translation:
The English word "lunatic," which in popular speech signifies a sufferer from any mental derangement, whether periodic or chronic, other than congenital idiocy, appears in the King James Version as a translation of the Greek word seleniazomai, in the two passages where it occurs. In the Revised Version (British and American) the word has very properly been displaced by the strictly accurate term "epileptic." This change is justified not only by the extra-Biblical usage (see Liddell and Scott, under the word), but clearly enough by Mt 17:15 (compare Mt 4:24), where epilepsy is circumstantially described.
2. Original Meaning:
The original meaning of the term seleniazomai, "moon-struck," is connected with the popular belief, widespread and of strange persistency, that the moon, in certain of its phases, is injurious to human beings, especially in the case of diseases of a periodic or remittent character. There are no data by which to determine whether, in the New Testament times, this particular word represented a living and active belief or had passed into the state of usage in which the original metaphor disappears, and the word simply indicates the fact signified without reference to the idea embodied in the etymology. We still use the word "lunatic" to signify a person mentally diseased, although we have long since ceased to believe in the moon's influence in such cases.
The Bible designates "madness," or alienation of mind, by various terms, all of which seem to be onomatopoetic. These various words seem to be derived from the strange and fierce or mournful cries uttered by the unfortunate victims of this dread malady. In De 28:34 the word "maddened" is meshugga`, participle of shagha` (compare also 1Sa 21:15). With this corresponds the word mainomai, in the New Testament. In 1 Sam 21:13 (Hebrew 14) the word is a form of the verb chalal, which is also a derivative from the sound indicated.
In certain cases, though by no means uniformly, madness is ascribed to demon-possession (Lu 8:26 f) . One is struck by the fact that mental derangement occupies a very small place in Scripture.
Louis Matthews Sweet