3 Nephi 22

3 Nephi 22:17

"No weapon that is formed against thee shall prosper..."

This is referring to persecution against the Church.  Throughout its history terrible things have happened. Though mobs don't storm against the Church today, there are many ideas against the Church that lead people astray. Things that may seem to harm the spread of the Gospel. Here though, Christ promises that none of these things will prosper.