Ross Geddes has pointed out (personal communication, 28 August 2005) that in all the printed editions of the Book of Mormon the second that-clause is treated as the complement to the verb believe (the result of there being no punctuation between the verb believe and the following that). Geddes argues that the angel makes two declarations, numbered above as 1 and 2: “many shall see greater things than these” and “these signs and wonders should come to pass upon all the face of this land” (the modal should here means ‘shall’). Each declaration has its own purpose, expressed by an immediately following subordinate clause initiated by “to the intent”: the righteous believe because the Lord reveals even greater events before they have happened, while the world will have no excuse for not believing after these events have actually occurred.
The verb believe that ends the subordinate clause “to the intent that they might believe” does not need any complement, as shown by the following example:
In fact, this later passage in the book of Helaman is saying the same thing as the first declaration here in Helaman 14:28. Thus there is a real need to separate believe in Helaman 14:28 from the following that with a comma—or even better, with a dash—to help the reader realize that the second that-clause parallels the first that- clause.
Summary: Place a dash before the second that-clause in Helaman 14:28 so that the reader will not interpret this clause as complementing the immediately preceding verb, believe, but instead as the start of a second that-clause that contrastively parallels the first that-clause.