As with the earlier comment about errors in the text, this statement in verse 17 requires that it be read in the context of the explanation about the errors. The errors are “the faults of a man.” However, Moroni states “we know no fault.” Because it is recorded by human hands, it is subject to the best efforts a human could make, and Moroni declares that is exactly what has happened.
Nevertheless, “God knoweth all things,” which means that God knows both the places where there might be faults, but also that the doctrine it teaches is correct. Since that doctrine comes from God, those who condemn the book are inadvertently condemning God.
Verse 18 is a variation on the problem of asking for signs. Moroni expects that the future readers might decide that they need more of a sign than the fact of its existence (and translation). They should be wary of tempting the Lord.
The intent of this set of verses (verses 17–22) is to declare to future readers that this is the work of God, and fulfills God’s promises (that the record of the Nephites would come forth).