Moroni creates a parallelism between judging the Book of Mormon rashly and being judged rashly (of God). This is a reworking of the New Testament concept of the parallelism between our actions and the judgment upon our actions. The Matthean base text is:
Matthew 7:1-2
1 Judge not, that ye be not judged.
2 For with what judgment ye judge, ye shall be judged: and with what measure ye mete, it shall be measured to you again.
Moroni may have been familiar with this concept, for it is also included in 3 Nephi 14:1-2. Moroni has taken that basic relationship, and applied it in a new context. This context is particularly interesting because it adds the idea of judging rashly. From the human standpoint, we certainly understand that a rash decision is one made quickly without attention to the full facts. While this does describe the ways in which many approach the Book of Mormon, it cannot accurately describe God’s justice, as God would never judge us rashly, or on insufficient data. The presence of the second “rashly” is dictated by the parallelism, not theology.