How might one know that the gospel is being taught? How can the church of the true believers be distinguished from those who believe some, but deny some? Moroni returns to the theme of the miracles. These miracles will be present, because true faith is present.
Reference: The concept that faith produces the miracles that will be the mark of the true church is an echo of the language of Mark:
Mark 16:17-18
17 And these signs shall follow them that believe; In my name shall they cast out devils; they shall speak with new tongues;
18 They shall take up serpents; and if they drink any deadly thing, it shall not hurt them: they shall lay hands on the sick, and they shall recover.
The language is obviously parallel. The meaning fits into Moroni’s discourse, but the language us influenced by the New Testament.