Are there unholy prophets?

An attributive modifier such as an adjective can be restrictive or non-restrictive. For example, in such phrases as Holy Land and holy water, the modifier holy is restrictive; it restricts the scope of reference to a particular land and a particular type of water, in contrast to other lands and ordinary water. In Holy Bible and Holy Trinity, the same modifier is non-restrictive; Bible and Holy Bible have the same scope of reference. Holy does not specify a particular type of Bible but adds extra or parenthetical information -- "the Bible (which by the way is holy)."

Some languages mark restrictiveness with grammatical rules. For example, in Spanish, a restrictive adjective generally comes after the noun it modifies (e.g. Tierra Santa, agua bendida), while a non-restrictive one comes before it (e.g. Santa Biblia, Santísima Trinidad).

In English, restrictiveness is often grammatically unmarked. In appositives and relative clauses, non-restrictiveness is marked by the use of commas, and restrictiveness can optionally be marked by the use of that rather than which or who, but there are no corresponding rules for adjectives. Holy Land and Holy Bible have the same grammatical form, the only distinction being that in restrictive phrases like the former, the adjective is usually stressed (HOLY Land), while the noun is typically stressed in non-restrictive phrases (Holy BIBLE); there are many exceptions to this, though (e.g. Holy GHOST, cf. Spanish Espíritu Santo), and in any case the distinction is invisible in writing.

This is all by way of preface to a discussion of holy prophets, a phrase that appears four times in the New Testament and a whopping 26 in the Book of Mormon. I had always assumed that this was a non-restrictive honorific, analogous to putting Saint before a saint's name, but in my most recent reading of Abinadi's words in Mosiah, I noticed this verse. The context is that Abinadi is explaining Isaiah's statement that the Suffering Servant (understood to be Christ) "shall see his seed."

Yea, and are not the prophets -- every one that has opened his mouth to prophesy, that has not fallen into transgression -- I mean all the holy prophets ever since the world began -- I say unto you that they are his seed (Mosiah 15:13).

The standard Spanish version of the Book of Mormon has the non-restrictive santos profetas here, but I think profetas santos is surely correct. As mentioned above, relative clauses with that are always restrictive in English. "Every one that has opened his mouth to prophesy, that has not fallen into transgression" implies that not all have opened their mouths, and that some have fallen into transgression. Abinadi is singling out a subset of prophets -- namely, holy prophets, defined as those who have opened their mouths to prophesy and have not fallen into transgression -- and saying that they, and not prophets more generally, can be considered the metaphorical "seed" of the Suffering Servant. (It is somewhat odd that Isaiah twice emphasizes that the Servant "opened not his mouth," while Abinadi identifies the Servant's seed among the prophets as those who do open their mouths.)

Not all prophets are holy; in fact, it must be extraordinarily difficult to be a holy prophet. The first temptation is to keep quiet and not share the message -- like Jonah before the whale interrupted his plans, or Jeremiah before the fire shut up in his bones compelled him. For those who do speak out, the temptation is to use one's status as prophet for self-serving ends.