In verse 12 Moroni had promised that readers who could overlook the record’s faults would receive “greater things.” Now he confirms that faults exist. The message comes from Yahweh, in whom there is no error. Nevertheless, the word of God comes through men, a conduit that allows “the faults of a man” to creep into Yahweh’s message. This message is so important for modern readers that Moroni repeats it in the final line of the Title Page: “And now, if there are faults they are the mistakes of men.”
However, “we know no fault,” Moroni explains, meaning that it contains no intentional errors. Mormon and Moroni know they are not perfect—hence, the ever-present possibility of error. But the implication is that, if they knew of a fault, they would have corrected it.
Reference: In his vision of Joseph’s day, Moroni has also seen how the Book of Mormon will be received. This warning to “he that condemneth” parallels the believer who will receive “greater things” (v. 12). In contrast, the condemner will be “in danger of hell fire.”
This phrase is Matthew 5:22, which Jesus repeats in 3 Nephi 12:22. The contexts are completely different, so the borrowing is purely lexical—a function of Joseph’s familiarity with the King James Version.