Helaman 6

Even though Mormon records how often the Nephites and Lamanite return to their wickedness -

"...like the dog to his vomit, or like the sow to her wallowing in the mire." (Book of Mormon | 3 Nephi 7:8)

He also records how dissenters, apostates, and sinners repent and return to the fold for we even read -

"...the Lamanites did come down into the land of Zarahemla, and did declare unto the people of the Nephites the manner of their conversion, and did exhort them to faith and repentance.
5 Yea, and many did preach with exceedingly great power and authority, unto the bringing down many of them into the depths of humility, to be the humble followers of God and the Lamb." (Book of Mormon | Helaman 6:4 - 5)

And even while there were still many that continued in wickedness -

"...the people of the church did have great joy because of the conversion of the Lamanites, yea, because of the church of God, which had been established among them. And they did fellowship one with another, and did rejoice one with another, and did have great joy." (Book of Mormon | Helaman 6:3)

This conversion again came about as they preached "with exceedingly great power and authority". This is priesthood authority and power and the power of the Holy Ghost.

We see that many of the wicked Nephites and Lamanites began to join the band of Gadiantion robbers. With this group that would lie, deceive, rob, plunder and murder, and live immoral, we find -

"...that when the Lamanites found that there were robbers among them they were exceedingly sorrowful; and they did use every means in their power to destroy them off the face of the earth." (Book of Mormon | Helaman 6:20)

But again, we learn the most effective way from the Lamanites at this time -

"...we see that the Lord began to pour out his Spirit upon the Lamanites, because of their easiness and willingness to believe in his words." (Book of Mormon | Helaman 6:36)

And so what did they do that was so successful?

"...the Lamanites did hunt the band of robbers of Gadianton; and they did preach the word of God among the more wicked part of them, insomuch that this band of robbers was utterly destroyed from among the Lamanites." (Book of Mormon | Helaman 6:37)