In the Book of Mormon text, the preposition of is possible, but rare, after the verb understand. Here in Alma 33:16 the original of means ‘concerning’ and could be thought of as adding a partitive sense to the direct object, thy mercies. Because of its strangeness in modern English, the committee for the 1920 LDS edition eliminated this instance of of in their editing for that edition. The change is intentional because it was marked in the committee copy.
There is one other example of “to understand of X” in the Book of Mormon; this other example has the same basic meaning as the one here in Alma 33:16 (namely, ‘to have comprehension of ’), yet in this case the of was not edited out of the 1920 edition:
There is an example of “to understand of X” in the King James Bible but with the meaning ‘to find out about’:
Despite the difference in meaning, the preposition of here, as with the Book of Mormon examples, means ‘concerning’. The online Oxford English Dictionary lists numerous examples of “to understand of X”, with either the meaning ‘to have comprehension of something’ (see definition 10b under the verb understand ) or the meaning ‘to find out about something’ (see definition 11). The OED also indicates that the use of the preposition of in these instances is either archaic or obsolete. Here I give one citation for each of these meanings (with original accidentals retained):
The critical text of the Book of Mormon will therefore accept the archaic use of “to understand of X” in Mosiah 1:5 and Alma 33:16.
Summary: Restore in Alma 33:16 the preposition of in the clause “because they will not understand of thy mercies”; such archaic usage occurred in Early Modern English and can be found elsewhere in the Book of Mormon text, namely in Mosiah 1:5: “that we might read and understand of his mysteries”.